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

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

Reverse the condition of the return of the build function to match the rest of Clang, then I think we're good. 

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