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

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@erichkeane Regarding "Still waiting on this?", I pushed a commit that addresses the fixme. I'll be addressing the rest of your review comments in a subsequent commit..

Also, if we are renaming `ExplicitInstantiationInfo::ExternLoc` to `ExternKeywordLoc`, should we rename the getters and setters to `getExternKeywordLoc`/`setExternKeywordLoc` as well?

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