[clang] [Clang] Implement CWG2598: Union of non-literal types (PR #78195)

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groundswellaudio wrote:

By looking at your tests on anonymous unions, I'd say that indirect fields are handled correctly (perhaps they're not included in the fields range?, otherwise the union-like class would not be considered literal) and that rather the implicitly generated special member functions of the anonymous union are the problems (and it's unclear to me why these would be needed at all, but perhaps it's simpler to generate them that to make a special case anonymous unions). 

https://github.com/llvm/llvm-project/pull/78195