[clang] [Clang] Unify interface for accessing template arguments as written for class/variable template specializations (PR #81642)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 commented:

Sounds good. My  instinct reaction to the title is that you're going to unify the annoying duplicated interfaces for the 5 specialization classes (function specialization, class/var (partial) specializations). But it is still good to merge these things.

https://github.com/llvm/llvm-project/pull/81642