Compute NLL loan scopes using the polonius model

[lqd]

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 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.