[clang] [Clang] Fix missing diagnostic for non-standard layout type in `offsetof` (PR #65246)

[Louis Dionne via cfe-commits]

ldionne wrote:

I think when the @llvm/pr-subscribers-libcxx group was pinged 2 weeks ago, most of us didn't have the right GH notifications set up yet and we didn't see it. Looking now.

https://github.com/llvm/llvm-project/pull/65246