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

[Louis Dionne via cfe-commits]

ldionne wrote:

@kasuga-fj Ok, let's take the diff I posted earlier in the comments and silence the warning in libc++ then. Once you do that, the libc++ test suite shouldn't fail with your patch anymore so your CI should be green. Then libc++ will be out of your way and it's up to the Clang folks whether this is worth doing.

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