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.

ronen/Halide,jiawen/Halide,kgnk/Halide,kgnk/Halide,ronen/Halide,psuriana/Halide,ronen/Halide,ronen/Halide,ronen/Halide,jiawen/Halide,ronen/Halide,kgnk/Halide,jiawen/Halide,psuriana/Halide,psuriana/Halide,ronen/Halide,kgnk/Halide,jiawen/Halide,kgnk/Halide,psuriana/Halide,kgnk/Halide,ronen/Halide,jiawen/Halide,psuriana/Halide,kgnk/Halide,psuriana/Halide,jiawen/Halide,jiawen/Halide,psuriana/Halide,kgnk/Halide