Automatic merge from submit-queue

Prep for not checking in generated, part 2/2

This PR is extracted from #25978 and build on #28578 - it is just the conversion related parts. All the Makefile 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