Add some sanity checks for LLVM for the CMake build.
Add some sanity checks for LLVM for the CMake build.


Former-commit-id: 9ce1d84a4b78e4121b8516e00bbe70ece357c58e
darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide