swift234

Use the constraint graph to isolate and solve the smallest connected component.

At each solution phase, construct a constraint graph and identify the
smallest connected component. Only solve for the variables in that
connected component, and restrict simplification to the constraints
within that connected component. This reduces the number of
visited-but-not-simplified constraints by ~2x when type-checking the
standard library.

Performance-wise, this is actually a regression (0.25s/8% when parsing
the standard library), because the time spent building the constraint
graph exceeds the time saved by the optimization above.

The hackaround in the standard library is due to
<rdar://problem/15168483>. Essentially, this commit changes the order
in which we visit type variables, causing the type checker to make
some very poor deduction choices.

The point of actually committing this is that it validates the
constraint graph construction and sets the stage for an actual
optimization based on isolating the solving work for the different
components.



Swift SVN r10672