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

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groundswellaudio wrote:

@Fznamznon 
@cor3ntin 
So https://github.com/llvm/llvm-project/pull/77753 doesn't solve the problem of an anonymous union not being literal when it should be, right? 

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