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

[Louis Dionne via cfe-commits]

ldionne wrote:

Note that you should probably rebase your patch onto `main` and force-push to update the PR. The CI for Clang now runs on Windows from GH PRs as well (it used not to when you submitted the patch, I think).

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