[clang] Improve error message for invalid lambda captures (PR #94865)

[via cfe-commits]

https://github.com/Sirraide commented:

This looks reasonable, but as was already pointed out, it needs a few tests, and also a release note (in `clang/docs/ReleaseNotes.rst`, probably in the ‘Improvements to Clang’s Diagnostics’ section or whatever it was called).

https://github.com/llvm/llvm-project/pull/94865