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

[Louis Dionne via cfe-commits]

ldionne 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.

The CI issues with the Benchmarks should be fixed now, so if you rebase onto `main` it shouldn't be a problem anymore

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