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

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

Changes to C++ DR tests look fine, save for one detail I left a comment for.

I'm a bit surprised that implicitly defined destructor of a union is deleted if default constructor is not trivial, but I it is discussed in the paper, mentioning that Core asked for this, so I guess it's fine.

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