[clang] [Clang][RAV] Simplify TraverseTemplateArgumentLocsHelper (PR #199131)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

FYI `getTemplateArgsAsWritten` is also used in explicit instantiations, and that's probably why you are seeing the assert not hold.

explicit instantiations are really quite similar to implicit ones, except they have different linkage, and they have some extra baggage, like some properties which really ought to appertain to the `ExplicitInstantiationDecl` alone.

I think with @16bit-ykiko 's work on ExplicitInstantiationDecl, we could simplify this and remove both explicit instantiation specialization kinds, and treat explicit instantiations just like implicit instantiations almost everywhere, except that we can use the `ExplicitInstantiationDecl` to calculate their linkage.

https://github.com/llvm/llvm-project/pull/199131