Also, factor out `do_mir_borrowck`, which is the code that actually
performs the MIR borrowck from within the scope of an inference context.
This change should be a pure refactoring.
This will be important in next commit, since the input types will be
tagged not with `'gcx` but rather `'tcx`. Also, using the region-erased,
lifted types enables better caching.
This falls naturally out of making drop elaboration work with `box`
expressions, which is probably required for sane MIR borrow-checking.
This is a pure refactoring with no intentional functional effects.
Turn `elaborate_drops` and `rustc_peek` implementations into MIR
passes that also live in `rustc_mir` crate.
Rewire things so `rustc_driver` uses the `ElaborateDrops` from
`rustc_mir` crate.
