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

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@mizvekov So, after some experimentation, I think the correct solution is my original change which augments `getPrimaryTemplate` and related functions to return the most recent declaration. Otherwise, the number of cases where we accidentally use the wrong definition of a function/class/variable template will be innumerable. 

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