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

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

> @groundswellaudio actually, that should be fixed by https://github.com/llvm/llvm-project/pull/77753
@Fznamznon Can you confirm?

Yes, it does make an error to go away. However the function `foo` from the [code](https://github.com/llvm/llvm-project/pull/78195#issuecomment-1895950521) won't be callable. If it is `constexpr` instead of `consteval`, then it is callable outside of constexpr context.

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