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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Aside from some nits, I think this looks reasonable to me. I'll leave the final sign-off to @erichkeane but if he doesn't approve in the next few days (WG21 meetings are upcoming so he may be busy), I'll come back to approve.

Thank you for all the help on this one! It turns out `good first issue` is really hard to determine accurately. ;-)

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