swift234

When ranking constraint solutions, consider anything that causes
a function conversion to be bad.

This encourages the type-checker to place conversions within
closures rather than outside; the test case here crashed in
SIL verification because of that.  (Yes, that means that there's
an underlying problem still when the function conversion is
required; that's tracked by rdar://15875305.)  But in general,
function conversions are likely to be expensive, and it's good
to avoid them when possible.

The setup work to add SK_FunctionConversion was accidentally
committed as part of r12813.

Swift SVN r12839