Automatic merge from submit-queue
Prep for not checking in generated, part 1/2
This PR is extracted from #25978 - it is just the deep-copy related parts. All the Makefile and conversion stuff is excluded.
@wojtek-t this is literally branched, a bunch of commits deleted, and a very small number of manual fixups applied. If you think this is easier to review (and if it passes CI) you can feel free to go over it again. I will follow this with a conversion-related PR to build on this.
Or if you prefer, just close this and let the mega-PR ride.
@lavalamp
42805f53b0