renumber regions in generators
This fixes#47189, but I think we still have to double check various things around how to treat generators in MIR type check + borrow check (e.g., what borrows should be invalidated by a `Suspend`? What consistency properties should type check be enforcing anyway around the "interior" type?)
Also fixes#47587 thanks to @spastorino's commit.
r? @pnkfelix