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

[Matheus Izvekov via cfe-commits]

================

----------------
mizvekov wrote:

After we fork for clang-20, we can entirely remove `frelaxed-template-template-args`, and so won't need to worry about testing that non-conforming mode. Would it be reasonable to postpone that until then?

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