[ConstraintSolver] Prioritize certain type variables while looking for bindings

Presence of some constraints (Subtype at least) requires a certain
contextual ranking of the type variables associated with them when
it comes to picking bindings, otherwise it might lead to no or
invalid solutions, because only a set of the bindings for the best
type variable is attempted.

Resolves: rdar://problem/22898292

lib/Sema/CSSimplify.cpp

@@ -1695,13 +1695,14 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
         }
 
         // If we have a binding for the right-hand side
-        // (argument type) don't try to bind it to the left-hand
-        // side (parameter type) directly, because their
-        // relationship is contravariant and the actual
-        // binding can only come from the left-hand side.
+        // (argument type used in the body) don't try
+        // to bind it to the left-hand side (parameter type)
+        // directly, because there could be an implicit
+        // conversion between them, and actual binding
+        // can only come from the left-hand side.
         addUnsolvedConstraint(
-            Constraint::create(*this, ConstraintKind::ArgumentConversion, type2,
-                               typeVar1, getConstraintLocator(locator)));
+            Constraint::create(*this, ConstraintKind::Equal, typeVar1, type2,
+                               getConstraintLocator(locator)));
         return SolutionKind::Solved;
       }