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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for working on this! I think the changes should come with a release note so users know about the fix. I did have a question on the design, but overall the direction seems sensible to me.

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