[Compiler-rt] Fix bug when considering CMake path returned by llvm-config.

The previous check was wrong because it only checks that the LLVM CMake
directory exists. However, it's possible that the directory exists but
the `LLVMConfig.cmake` file does not. When this happens we would
incorectly try to include the non-existant file.

To fix this we make the check stricter by checking that the file
we want to include actually exists.

This is a follow up to fd28517d.

rdar://76870467