Auto merge of #29886 - michaelwoerister:mir-erase-regions, r=nikomatsakis

Auto merge of #29886 - michaelwoerister:mir-erase-regions, r=nikomatsakis

This change adds a `MirPass` erasing all early-bound regions from MIR, right before storing it in the MIR map. I've added some assertions at neuralgic points in `trans::mir` doing cheap checks whether region have actually been erased.

Here are some assumptions that I worked under:
- AdtDef references stay untouched. It's the `Substs` accompanying them that need to be handled (e.g. in `AggregateKind::Adt`).
- We can't really get rid of late-bound regions at this point because there is no version `BareFnTy` (for example) that comes without one. These still have to be handled on demand in trans.

Are this assumptions right?

r? @nikomatsakis