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

[James Y Knight via cfe-commits]

jyknight wrote:

I'm not sure I understand what happened here, since `_Alignas` did work properly before this change, right? How/why are `_Alignas` and `alignas` acting differently from eachother in C23 mode?

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