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

lib/Sema/CSSimplify.cpp

@@ -946,6 +946,14 @@ commit_to_conversions:
     return SolutionKind::Error;
   }
 
+  // Okay, we need to perform one or more conversions.  If this
+  // conversion will cause a function conversion, score it as worse.
+  // This induces conversions to occur within closures instead of
+  // outside of them wherever possible.
+  if (locator.isFunctionConversion()) {
+    increaseScore(SK_FunctionConversion);
+  }
+
   // Where there is more than one potential conversion, create a disjunction
   // so that we'll explore all of the options.
   if (potentialConversions.size() > 1) {