[clang] [Clang][Sema] Diagnose use of template keyword after declarative nested-name-specifiers (PR #78595)

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@rupprecht This is intended behavior. We previously did not check qualified declarations of explicit/partial specializations of class templates for redundant qualification & invalid nested-name-specifiers (e.g. ones that begin with `decltype`). We do now, hence the new warning. 

https://github.com/llvm/llvm-project/pull/78595