swift234

[Constraint solver] Initial work on a less horrible solver.

This solver handles most of what the old solver does (enabling it by
default only causes 9 test failures), but is radically simpler and
already significantly faster (~50% slower than the old-old type
checker on swift.swift). There are some egregious hacks here to still
be eliminated (see FIXMEs), but it's a start.


Swift SVN r5272