[clang] Infer compilation directory in driver
When building with -fdebug-compilation-dir/-fcoverige-compilation-dir,
infer the compilation directory in clang driver, rather than try to
fallback to VFS current working directory lookup during CodeGen. This
allows compilation directory to be used as it is passed via cc1 flag and
the value can be empty to remove dependency on CWD if needed.
Reviewers: adrian-prantl, dwblaikie
Reviewed By: adrian-prantl, dwblaikie
Pull Request: https://github.com/llvm/llvm-project/pull/150112
