[llvm] [Support] Fix thread safety issue in raw_null_ostream (PR #162787)

[Michal Paszkowski via llvm-commits]

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

LGTM! Let's wait for the builds and checks to finish. Tagging @dwblaikie and @kuhar (maintainers for this part of the project) for visibility/review.

https://github.com/llvm/llvm-project/pull/162787