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

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philnik777 wrote:

Assuming that Clang doesn't plan on optimizing anything based on this, I think it should be fine to just silence the warning. We could also add a `__datasizeof` builtin in a separate patch if we want. That would probably be less error-prone and cheaper than our current solution. FWIW we'll need some version of this when we want to replace `__compressed_pair`, so a cheaper solution would probably be a good idea.

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