[clang] [Clang] Produce determenistic hash for anonymous namespaces. (PR #194542)

[Aaron Ballman via cfe-commits]

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

LGTM but this should come with a release note in `clang/docs/ReleaseNotes.rst` so users know about the fix.

https://github.com/llvm/llvm-project/pull/194542