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

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

Thanks for working on this. Given that @brevzin started a PR, make sure he is fine with you taking over that work.
There is a C++ meeting in a couple of weeks where P3726R1 might be adopted. I plan to review afterwards.

The comments should either quote the standard directly, or not use that format, as we expect actual format to follow a section. I think it's fine to say `(see C++26 [foo.bar])` in places you are not quoting the standard directly.

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