Check result of llvm-config call in CMake build
Check result of llvm-config call in CMake build

If it fails, crater with a fatal error; otherwise, we can fail much
later in the build process with inscrutable-seeming link errors.


Former-commit-id: 85b694cf50f765ad9d51ef14374288ec715a14ed
darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide