[clang] [Clang] Fix type elision and diagnostic text formatting for nested template diffing (PR #198976)

[Mariya Podchishchaeva via cfe-commits]

https://github.com/Fznamznon commented:

So that basically is unchanged PR made by someone else with some test additions. It would be great to get a confirmation from the original author that they are ok with this.
Was AI used in preparation of this PR? If yes, please refer to https://llvm.org/docs/AIToolPolicy.html and disclose AI usage. Also It would be great to see a test that involves originally reported reproducer.

https://github.com/llvm/llvm-project/pull/198976