04145943a2
MIR EndRegion Statements (was MIR dataflow for Borrows) This PR adds an `EndRegion` statement to MIR (where the `EndRegion` statement is what terminates a borrow). An earlier version of the PR implemented a dataflow analysis on borrow expressions, but I am now factoring that into a follow-up PR so that reviewing this one is easier. (And also because there are some revisions I want to make to that dataflow code, but I want this PR to get out of WIP status...) This is a baby step towards MIR borrowck. I just want to get the review process going while I independently work on the remaining steps.