-Zvalidate-mir: check that all unwind edges go to `cleanup` blocks

[jonas-schievink]

Blocks that are part of unwinding code always need to marked as cleanup. This seems like an easy thing to check in the validation pass. I believe this would have caught a bug in the generator transform (a pretty long time ago).

This issue has been assigned to @DoctorN via this comment.

[RalfJung]

Good point! Moreover, cleanup block terminators should also only jump to cleanup blocks, and other terminators should only jump to non-cleanup blocks. Basically, unwind edges must be the only ones that cross the "cleanup to non-cleanup" boundary.

(Miri is already checking this.)

[cofibrant]

@rustbot claim