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

[via cfe-commits]

bgra8 wrote:

@sdkrystian we need a way to disable the new check so we can do the code fixes for our large codebase while still using the old compiler.

As the patch description suggests the correct code is rejected by the current `clang` so we have no path forward here. Can you please add a  flag to allow switching to the old behavior?

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