[clang] [clang][Parser] "Better" error messages for invalid template template (PR #95726)

[via cfe-commits]

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

LGTM after fixing two more minor things I only noticed just now.

Also, this needs a release note (in `clang/docs/ReleaseNotes.rst`, probably in the section ‘Improvements to Clang’s Diagnostics’ or whatever it was called).

https://github.com/llvm/llvm-project/pull/95726