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

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@kimgr The linked code seems to only be concerned with the `SourceLocation` of the `Decl` (which should be the same `SourceLocation` returned by `Decl::getLocation`), and the template arguments as written (which can be accessed via `getTemplateArgsAsWritten` per this patch). I would imagine the fix here would be to handle explicit instantiations of class templates the same what you handle explicit instantiations of function templates (which also do not store a `TypeLoc` but can have their template arguments as written accessed in a similar manner).

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