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

[Krystian Stasiowski via cfe-commits]

sdkrystian wrote:

@cor3ntin I changed the diagnostic to a pedantic warning and added a release note. 

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

We don't check for the template keyword when parsing an _elaborated-type-specifier_ without a _nested-name-specifier_, so such a declaration is parsed as an unnamed class. I feel that it's beyond the scope of this PR anyways, as this is only concerned with qualified declarations. 

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