[clang] [clang] Fix false positive with -Wdocumentation and explicit instanti… (PR #178223)

[Carlos Galvez via cfe-commits]

carlosgalvezp wrote:

The reason why this problem only shows up for functions and not for classes is that for classes, the explicit instantiation does **not** have the `FullComment` attached to it in the AST. This behavior seems inconsistent. Should this problem rather be fixed in the AST instead?

Example: https://godbolt.org/z/vsov5GMYa

https://github.com/llvm/llvm-project/pull/178223