[Clang][Sema] Differentiate between partial/explicit specializations when...

[Clang][Sema] Differentiate between partial/explicit specializations when diagnosing unexpanded packs (#72015)

This adds `UnexpandedParameterPackContext::UPPC_ExplicitSpecialization`
and passes it to `DiagnoseUnexpandedParameterPack` when checking
class/variable template explicit specializations.

Right now we don't check for unexpanded packs in function template
explicit specializations at all, so I will be addressing that in a
followup PR.