This is the script I currently use on OS X to get a working "swift-dev.xctoolchain" out of a built Swift. It isn't designed to work for anyone but me, but it should be easy to adapt. I always run this after every build, and then use `TOOLCHAINS=swift-dev swift build` (etc) to use the development compiler.

update-swift-dev

#!/bin/sh

set -e

if [ -z "${SWIFTPM_CONFIGURATION}" ]; then
  SWIFTPM_CONFIGURATION=.bootstrap/lib/swift/pm/4
fi

if [ -z "${CONFIGURATION}" ]; then
    CONFIGURATION=release
fi

if [ -z "${SWIFT_SOURCE_PATH}" ]; then
    SWIFT_SOURCE_PATH=~/Desktop/Swift
fi

if [ -z "${XCODE_PATH}" ]; then
  XCODE_PATH=$(xcode-select -p)
fi

# Create the development toolchain.
rm -rf ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain
cp -r ${XCODE_PATH}/Toolchains/XcodeDefault.xctoolchain ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain

# Rename and expand the Info.plist
mv ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/ToolchainInfo.plist ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/Info.plist
if ! /usr/libexec/PlistBuddy -c "Delete :Identifier" "${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/Info.plist" > /dev/null; then
    echo "error: unable to write to ToolchainInfo.plist 1"
    exit 1
fi

if ! /usr/libexec/PlistBuddy -c "Add :CFBundleIdentifier string 'com.cfi.swift.toolchain.swift-dev'" "${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/Info.plist" > /dev/null; then
    echo "error: unable to write to ToolchainInfo.plist 1"
    exit 1
fi

if ! /usr/libexec/PlistBuddy -c "Add :Aliases array" "${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/Info.plist" > /dev/null; then
    echo "error: unable to write to ToolchainInfo.plist 2"
    exit 1
fi

if ! /usr/libexec/PlistBuddy -c "Add :Aliases:0 string 'swift'" "${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/Info.plist" > /dev/null; then
    echo "error: unable to write to ToolchainInfo.plist 3"
    exit 1
fi

if ! /usr/libexec/PlistBuddy -c "Add :CompatibilityVersion integer 2" "${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/Info.plist" > /dev/null; then
    echo "error: unable to write to ToolchainInfo.plist 4"
    exit 1
fi

if ! /usr/libexec/PlistBuddy -c "Add :CompatibilityVersionDisplayString string 'Xcode 8.0'" "${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/Info.plist" > /dev/null; then
    echo "error: unable to write to ToolchainInfo.plist 5"
    exit 1
fi

INFO_PLIST_DATE=$(date)
if ! /usr/libexec/PlistBuddy -c "Add :CreatedDate date ${INFO_PLIST_DATE}" "${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/Info.plist" > /dev/null; then
    echo "error: unable to write to ToolchainInfo.plist"
    exit 1
fi

if ! /usr/libexec/PlistBuddy -c "Add :DisplayName string 'Swift 4.0 Release'" "${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/Info.plist" > /dev/null; then
    echo "error: unable to write to ToolchainInfo.plist"
    exit 1
fi

if ! /usr/libexec/PlistBuddy -c "Add :ShortDisplayName string 'Swift 4.0 Release'" "${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/Info.plist" > /dev/null; then
    echo "error: unable to write to ToolchainInfo.plist"
    exit 1
fi

if ! /usr/libexec/PlistBuddy -c "Add :Version string '4.0'" "${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/Info.plist" > /dev/null; then
    echo "error: unable to write to ToolchainInfo.plist"
    exit 1
fi

# Install the `swift` provided binaries.
rsync -ar \
      --link-dest ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/ \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swift \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swiftc \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swift-demangle \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/bin
# Install the `swiftpm` provided binaries.
rsync -ar \
      --link-dest ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swiftpm-macosx-x86_64/${CONFIGURATION}/ \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swiftpm-macosx-x86_64/${CONFIGURATION}/swift-build \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swiftpm-macosx-x86_64/${CONFIGURATION}/swift-package \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swiftpm-macosx-x86_64/${CONFIGURATION}/swift-test \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/bin/
# Install the `llbuild` provided binaries.
rsync -ar \
      --link-dest ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/llbuild-macosx-x86_64/bin/ \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/llbuild-macosx-x86_64/bin/swift-build-tool \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/bin/
# Install fake versions of `lldb` and `lldb-mi`.
rsync -ar \
      --link-dest ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/llbuild-macosx-x86_64/bin/ \
      $(which false) \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/bin/lldb
rsync -ar \
      --link-dest ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/llbuild-macosx-x86_64/bin/ \
      $(which false) \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/bin/lldb-mi
# Install the `swiftpm` runtime libraries.
mkdir -p ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/lib/swift/pm/
rsync -ar \
      --link-dest ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swiftpm-macosx-x86_64/${CONFIGURATION}/ \
${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swiftpm-macosx-x86_64/${SWIFTPM_CONFIGURATION}/libPackageDescription.dylib \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swiftpm-macosx-x86_64/${SWIFTPM_CONFIGURATION}/PackageDescription.swiftmodule \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/lib/swift/pm/
# Install the `clang` runtime data.
rsync -ar \
      --link-dest ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/llvm-macosx-x86_64/lib/libclang.dylib \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/llvm-macosx-x86_64/lib/libclang.dylib \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/lib/

# Install the standard libraries.
mkdir -p ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/lib/swift/macosx/
rsync -a \
      --link-dest ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/swift/macosx/ \
      $(ls ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/swift/macosx/*.dylib | \
               grep -v SwiftPrivate | \
               grep -v StdlibUnittest | \
               grep -v SwiftReflectionTest | \
               grep -v SwiftExperimental) \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/lib/swift/macosx/
rsync -a \
      --link-dest ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/swift/macosx/x86_64/ \
      $(ls ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/swift/macosx/x86_64/*.swiftdoc | \
               grep -v SwiftPrivate | \
               grep -v StdlibUnittest | \
               grep -v SwiftReflectionTest | \
               grep -v SwiftExperimental) \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/lib/swift/macosx/x86_64/
rsync -a \
      --link-dest ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/swift/macosx/x86_64/ \
      $(ls ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/swift/macosx/x86_64/*.swiftmodule | \
               grep -v SwiftPrivate | \
               grep -v StdlibUnittest | \
               grep -v SwiftReflectionTest | \
               grep -v SwiftExperimental) \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/lib/swift/macosx/x86_64/
rsync -a \
      --link-dest ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/swift/macosx/x86_64/ \
      $(ls ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/swift/macosx/x86_64/*.apinotesc | \
               grep -v SwiftPrivate | \
               grep -v StdlibUnittest | \
               grep -v SwiftReflectionTest | \
               grep -v SwiftExperimental) \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/lib/swift/macosx/x86_64/
rsync -ar --delete-excluded \
      --link-dest ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/sourcekitd.framework/ \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/sourcekitd.framework/ \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/lib/sourcekitd.framework/
# Install the `clang` symlink.
rsync -ar \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/swift/clang \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/lib/swift/clang
rsync -ar \
      --link-dest ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/swift/shims/ \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/swift/shims/ \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/lib/swift/shims/
rsync -a \
      --link-dest ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/ \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-macosx-x86_64/lib/lib*Demangle.dylib \
      ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/lib/

# Copy over `swift-stdlib-tool`.
cp $(xcrun --find swift-stdlib-tool) \
   ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain/usr/bin/

read -p "Would you like to move this to /Library/Developer/Toolchains; you must restart Xcode: (y/n)? " answer
case ${answer:0:1} in
  y|Y )
    sudo mkdir -p /Library/Developer/Toolchains;
    sudo rm -rf /Library/Developer/Toolchains/swift-dev.xctoolchain
    sudo cp -r ${SWIFT_SOURCE_PATH}/build/Ninja-ReleaseAssert/swift-dev.xctoolchain /Library/Developer/Toolchains/swift-dev.xctoolchain;
  ;;
  * )
    exit
  ;;
esac