swift234

(Optionally) Evolve the constraint graph during simplification.

Rather than building the constraint graph after simplification and
then never modifying it, build the constraint graph before
simplification and update it as simplification adds/removes
constraints. This is a step toward two separable performance goals:
(1) having a single constraint graph that evolves over the lifetime of
the constraint system, rather than building a new constraint graph at
each solver step, and (2) using the constraint graph to implement a
worklist traversal during simplification, so we only re-simplify those
constraints that might be affected by a choice the solver makes.

This behavior is currently optional with an ugly in-source flag
because while it works, it's a rather painful performance regression
that I don't want to subject everyone to. I'll turn it on for everyone
once it's a win, which should be after either (1) or (2) above is
implemented.



Swift SVN r10924