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

[Fangrui Song via cfe-commits]

MaskRay wrote:

> This commit will be authored by 30379873+carlocab at users.noreply.github.com

"Squash and merge" uses the PR author's github's setting. The commit email is completely ignored (unless there is another contributor, which would lead to a Co-authored-by:)

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