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

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cor3ntin wrote:

Thank for this patch 

I think this might be better done as a pedantic warning.
IE, I am concerned that this could break existing code, similarly to https://github.com/llvm/llvm-project/issues/78031#issuecomment-1891475606

I wonder if we should add tests for https://cplusplus.github.io/CWG/issues/1707.html in this patch.

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