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

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

Awesome, thanks!

I will give a review later, but just want to note a different (but much smaller) PR touching the same area: https://github.com/llvm/llvm-project/pull/102922

Just to keep in mind that the idea is to later further simplify `getTemplateInstantiationArgs` by removing the DeclContext parameter.

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