[clang] Thread Safety Analysis: Warn when using negative reentrant capability (PR #141599)

[Marco Elver via cfe-commits]

melver wrote:

> Thanks for the new diagnostic! I think you should also add a release note to `clang/docs/ReleaseNotes.rst` so users know about the new diagnostic group.

Added and updated the ThreadSafetyAnalysis.rst document as well.

PTAL.

https://github.com/llvm/llvm-project/pull/141599