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

[James Y Knight via cfe-commits]

https://github.com/jyknight approved this pull request.

Thanks, LGTM. I like this version.

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?

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