[clang] [clang-tools-extra] [clang] Implement CWG2398 provisional TTP matching to class templates (PR #94981)

[via cfe-commits]

https://github.com/cor3ntin approved this pull request.

Only nits at this point.
Sorry the review took this long, it was a pretty large PR.
Thanks for working on it!

https://github.com/llvm/llvm-project/pull/94981