[clang] Reapply "[Clang][Sema] Refactor collection of multi-level template argument lists (#106585)" (PR #111173)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

I am not sure I can offer a better option here. The global change seems odd, and I think it may run into other problems.

One option would be to do the big hammer approach as you suggest, but then afterwards back out and then try to nail the places where the most recent declaration is actually required. This way, the patch which needs to be reverted is much smaller, causing less churn.
Another option would be to introduce another helper `getPrimaryTemplateAsWritten`, and then gradually replace `getPrimaryTemplate` uses.

https://github.com/llvm/llvm-project/pull/111173