Compute NLL loan scopes using the polonius model
For a *location-insensitive* analysis (that is, without expressiveness improvements for users yet), this PR implements loans going out of scope using reachability and liveness, rather than checking if the issuing region's values contain a given CFG point. This is equivalent to NLL scopes and computes the same data.
r? `@matthewjasper`
A couple of notes:
- there are some assumptions about SCC representatives, placeholders, free regions, and member constraints that I believe hold, and they're documented in the code
- this passes all the UI tests with `-Zpolonius=next` -- the perf is [not terrible](https://github.com/rust-lang/rust/pull/112432#issuecomment-1749685862) and there are a bunch of ways to improve it in the future.
- there's a fixme left, hopefully Matthew you know a clean way to get the information it mentions.
c1691db366