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

[David Blaikie via cfe-commits]

dwblaikie wrote:

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

Not sure what the conventions are - but I rather like the normalization of having the explicit group, better chance of finding the group/reusing it maybe? So i'll leave it there for now, but if you feel strongly/other folks chime in - happy to change it in a follow-up commit. (& good to understand that explicit groups aren't necessary/the inline group definition is a valid option, though)

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