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

[Erich Keane via cfe-commits]

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

Thanks for the ping, I'd forgotten about this one!  I'm happy for the most part, I have 1 question and a couple nits.

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