[clang] [clang][deps] Make dependency directives getter thread-safe (PR #136178)

[Cyndy Ishida via cfe-commits]

https://github.com/cyndyishida approved this pull request.

LGTM, though you may want to wait a bit longer for someone more familiar to also take a look. 

https://github.com/llvm/llvm-project/pull/136178