Don't infer regions in type checking

[matthewjasper]

This is a tracking issue for avoiding region inference in item body type checking. The goal would be to use ReErased where we currently use ReVar as much as possible.

The motivations for this are:

• It would avoid doing region inference twice (once in typeck, once in borrowck).
• It might improve caching since there are fewer distinct types.
• It ensures that region inference cannot affect later passes.

cc @nikomatsakis

[matthewjasper]

The first steps towards this are #67681 and #65232

Some further steps are:

• Erase regions in writeback.rs (Erase regions in writeback #69189)
• Don't infer regions when checking function/const/static bodies, gated on -Zborrowck=mir
• Don't visit expressions in regionck. This requires some work to ensure that we don't regress error messages.

Some related work would be

• Remove ReScope ( Remove ReScope #72362)
• Ensure that MIR doesn't contain unerased free regions (except for borrowck's copy)