MCVE of "error: in auto-import: failed to get module 'foo' from AST context"

.gitignore

.DS_Store
/.build
/Packages
/*.xcodeproj

main.swift

import LLVM_C.Core

func foo(_ a: Int) {
    let b = a * a
    print(b)
}

foo(4)

Makefile

all:
	@swift build \
		-Xcc -I`llvm-config --includedir` \
		-Xlinker -L`llvm-config --libdir` \
		-Xlinker -rpath -Xlinker `llvm-config --libdir`

Package.swift

import PackageDescription

let package = Package(
    name: "foo",
    dependencies: [
        .Package(url: "https://github.com/rxwei/LLVM_C", majorVersion: 1, minor: 0)
    ]
)