[clang] Template Diagnostic Improvements (PR #99933)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov commented:

Hi.

The change looks good.

There are some other examples of bad diagnostics in that issue, and it would be ideal if they are preserved in case this PR closes that issue.

Please also add an entry to `clang/docs/ReleaseNotes.rst`.

https://github.com/llvm/llvm-project/pull/99933