[NLL] Add false edges out of infinite loops
Resolves#46036 by adding a `cleanup` member to the `FalseEdges` terminator kind. There's also a small doc fix to one of the other comments in `into.rs` which I can pull out in to another PR if desired =)
This PR should pass CI but the test suite has been relatively unstable on my system so I'm not 100% sure.
r? @nikomatsakis