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

[via cfe-commits]

kasuga-fj wrote:

Previous CI failures due to `offsetof` warnings have been resolved. Thank you for your cooperation! The current CI error (libcxx-ci Benchmark) does not seem to be caused by this patch (other PRs seem to be failing in the same one), so I'll wait until it's fixed.

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