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

[Fangrui Song via cfe-commits]

MaskRay wrote:

Note: you are using github's private email address feature, but llvm-project prefers to show the real email address. Actually, there is a bot detecting the issue and making a message, though it has some issues and does not trigger on this PR.

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