[clang] Fix a regression with alignas on structure members in C (PR #98642)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Thanks, LGTM. I like this version.

Thank you for the excellent review, I think we came to a better approach in the end!

> Did this change also fix any other bugs? Since it looks like we were previously not skipping ProcessDeclAttribute for `[[]]` attributes in C23 mode, when IncludeCXX11Attributes == false, and we were supposed to have been?

Theoretically yes, but practically no. We don't have any attributes in C23 that I think would be impacted by this except for `alignas`. At least, I've not been able to induce a difference in behavior.

https://github.com/llvm/llvm-project/pull/98642