Build LLVM/Clang on Linux/macOS from scratch (CMake required)

build.sh

#!/usr/bin/env sh
echo About to build LLVM/Clang.. Are you sure \(y/N\)? 
read consent
if [ $consent != 'y' ] && [ $consent != 'Y' ]; then
	echo Aborted
	exit 1
fi

echo Start downloading > time.log
date >> time.log
echo "Downloading llvm source.."
svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm > /dev/null
echo "Downloading clang source.."
cd llvm/tools > /dev/null
svn co http://llvm.org/svn/llvm-project/cfe/trunk clang > /dev/null
cd ../.. > /dev/null
echo "Downloading clang extra tools source.."
cd llvm/tools/clang/tools > /dev/null
svn co http://llvm.org/svn/llvm-project/clang-tools-extra/trunk extra > /dev/null
cd ../../../.. > /dev/null
echo "Done downloading."
date >> time.log

echo Start pre-building >> time.log
date >> time.log
mkdir build
cd build > /dev/null
cmake -G "Unix Makefiles" ../llvm
date >> ../time.log

echo Start building >> ../time.log
date >> ../time.log
make # builds both LLVM and Clang for debug mode.
     # For subsequent Clang development, you can just run: make clang.
date >> ../time.log

echo Done. && cat ../time.log
# If you intend to use Clang's C++ support, use the -DGCC_INSTALL_PREFIX 
# cmake option to tell Clang where the gcc containing the desired libstdc++
# is installed

# Run the testsuite: make check-clang

Now you can build LLVM/Clang using GN + Ninja as well, reportedly much faster, but is not official: llvm/utils/gn
GN and Ninja were developed to facilitate Chromium dev.