[clang] [Clang][Sema] Diagnose variable template explicit specializations with storage-class-specifiers (PR #93873)

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alexfh wrote:

Specifically, the problem is that we can't fix code that corresponds to case # 1 from the description, since it produces code corresponding to pattern # 2, which can't compile with current clang. Shipping fixes together with the compiler is not an option for us (and probably for many other users), so it should be possible to tell the new Clang to accept pattern # 1, i.e. it should be a warning (included into `-Wall`, probably) rather than a hard error for at least one compiler release.

https://github.com/llvm/llvm-project/pull/93873