[clang] [Clang] Ensure correct template parameter depth for abbreviated templates (PR #209693)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

> This change needs a release note. Please add an entry to `clang/docs/ReleaseNotes.md` in the section the most adapted to the change, and referencing any Github issue this change fixes. Thanks!

FYI I'm going to backport it because the assertion is introduced in clang 23 - we should have already suffered from the incorrect depth earlier but nothing fired.

https://github.com/llvm/llvm-project/pull/209693