swift234

Start reworking our handling of overloading in the constraint
system. Rather than creating child constraint systems during
constraint generation (which is a terrible idea), simply create an
overload set that has a bunch of choices. When the solver decides to
explore the consequences of picking one of the overloads from the set,
it will create the appropriate child system and inject the consrtaint
into that system.

No visible change of behavior, since none of this stuff is wired up
yet anyway.


Swift SVN r2667