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

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@cor3ntin Addressed your comments. I also added the error to a diagnostics group (`template-in-declaration-name`)

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