[clang] [clang] Improve `_Alignas` on a `struct` declaration diagnostic (PR #65638)

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https://github.com/cor3ntin commented:

LGTM modulo nitpick.
Maybe @AaronBallman will want to look at it too.
Do we want a release notes for this?

https://github.com/llvm/llvm-project/pull/65638