[clang] Clang: Add warning flag for storage class specifiers on explicit specializations (PR #96699)

[Krystian Stasiowski via cfe-commits]

https://github.com/sdkrystian approved this pull request.

Minor nit: Probably don't need to explicitly define a diagnostic group, since it's only ever used for this diagnostic. 

https://github.com/llvm/llvm-project/pull/96699