[clang] [Clang] Implement P3074R7: trivial unions (C++26) (PR #185886)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt commented:

I can't look at this too hard right now, but I _think_ this should be ok with pointer auth atm. There is an option that's not yet upstreamed that will impact this, but I think the implementation already neuters the "this type can be zero-inited" logic that influences this logic.

*thinking*

Does this have any impact on copy/move semantics? (Again, no time to read spec atm)

https://github.com/llvm/llvm-project/pull/185886