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

[via cfe-commits]

cor3ntin wrote:

Generally looks good, thanks for working on this

I'd like to see more tests in `clang/test/Sema/alignas.c` - we don't have a test for `struct _Alignas(int)  T` either apparently.
Given this fixes a bug, it could benefit from a release note in `clang/docs/ReleaseNotes.rst`

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