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

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kasuga-fj wrote:

The CI failed because `libcxx` uses `offsetof`.
https://github.com/kasuga-fj/llvm-project/blob/b9c767a6a128f57858273eb43d4383e83d0d086b/libcxx/include/__type_traits/datasizeof.h#L50
I investigated but couldn't figure out where to fix it (Should I change the compile time options?). Could someone please help me? It may be unrelated, but I am also concerned that "libc++ test suite in C++03" failed while "libc++ test suite in C++26" succeeded.

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