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

[via cfe-commits]

cor3ntin wrote:

> Is this a potentially breaking change since we have expanded what types are considered literals and already existing code may observe this fix?

Some expression will become constant expressions and presumably that can be SFINAE upon, but given that this is a DR, I don't think we are affecting conformance and I don't think we make strong sfinae guarantee.

GCC is conforming with that DR in all language mode since 10.0 
https://compiler-explorer.com/z/hKnThPx1T

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