Auto merge of #53873 - nikomatsakis:nll-universe-subtyping-and-pattern-ascription...

support ascription for patterns in NLL

This implements the strategy outlined in [this comment](https://github.com/rust-lang/rust/issues/47184#issuecomment-416669986):

- We first extend the NLL subtyping code so it can handle inference variables and subtyping.
- Then we extend HAIR patterns with type ascription.
- Then we treat the type `T` in `let pat: T = ...` as an ascription.

Before landing, a few things:

- [x] Fix the WF rule bug (filed a FIXME https://github.com/rust-lang/rust/issues/54105)
- [x] Fix an ICE I encountered locally around bound regions, or else file a follow-up
- [x] More tests probably =)

r? @pnkfelix