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

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

@cor3ntin , perhaps a dumb question, but https://github.com/llvm/llvm-project/pull/77753 should only help in post C++20 modes since only there destructor can be `constexpr`. How about older versions? Clang will still emit an error that the type from https://github.com/llvm/llvm-project/pull/78195#issuecomment-1895950521 is not literal in C++17 for example.

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