[clang] [Clang][Driver] Support relative paths for CLANG_CONFIG_FILE_SYSTEM_DIR (PR #110962)

[Carlo Cabrera via cfe-commits]

carlocab wrote:

Thanks, done! I thought that was preferred given the handling of `CLANG_CONFIG_FILE_USER_DIR` below it.

https://github.com/llvm/llvm-project/pull/110962