[clang] [Clang][Sema] Refactor collection of multi-level template argument lists (PR #106585)

[Erich Keane via cfe-commits]

erichkeane wrote:

I agree I think with your take on temp.expl.spec, we run into this in quite a few places.  I'd like to hear from the others, but this is a paragraph I've used in the past to justify a lot of this sort of diagnostics.

https://github.com/llvm/llvm-project/pull/106585