[clang] [Clang][Sema] Synthesize a memcpy body for defaulted union assignment (PR #206579)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin commented:

I am happy with this approach when @erichkeane is.

--

This change needs a release note.
Please add an entry to `clang/docs/ReleaseNotes.md` in the section the most adapted to the change, and referencing any Github issue this change fixes. Thanks!

https://github.com/llvm/llvm-project/pull/206579