Split exact-subclass and bindable-to-subclass queries.

In many places, we're interested in whether a type with archetypes *might be* a superclass of another type with the right bindings, particularly in the optimizer. Provide a separate Type::isBindableToSuperclassOf method that performs this check. Use it in the devirtualizer to fix rdar://problem/24993618. Using it might unblock other places where the optimizer is conservative, but we can fix those separately.

lib/AST/ProtocolConformance.cpp

@@ -521,7 +521,7 @@ found_inherited:
                                  subMap, conformanceMap);
   }
   assert((getType()->isEqual(foundInherited->getType()) ||
-          foundInherited->getType()->isSuperclassOf(getType(), nullptr))
+          foundInherited->getType()->isExactSuperclassOf(getType(), nullptr))
          && "inherited conformance does not match type");
   return foundInherited;
 }