swift234

[Constraint solver] Introduce a *trivial* 'meet' operation for types.

Stub out a simplistic 'meet' operation for types that currently only
handles finding the meet of two class types. Use it to optimize the
constraint solver ever so slightly: when we see a type variable whose
constraints state that it is a supertype of two different concrete
types, we attempt to compute their meet and, if we find it, only
produce one potential binding that is the meet itself.

Note that this is an extremely poor implementation of this concept
that is meant to address a specific regression being introduced by the
defaulting of collection literal element types. A real implementation
would, at the very least:

* Implement a proper 'meet' that covers all subtyping in the language.
* Distinguish between "I don't know if there is a meet" and "I am
  absolutely certain that there is no meet".
* Collapse the constraints themselves to reduce the number of
  constraints in the system, rather than just the number of type
  variable bindings we attempt.