[ConstraintSystem] Allow generic parameters to bind to holes only in diagnostic mode

If generic parameter gets opened during regular solving it cannot
be bound to a hole, that can only happen in diagnostic mode, so
let's not even try. Doing so also makes sure that there are no
inference side-effects related to holes because bindings are ranked
based on their attributes.

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -1866,11 +1866,14 @@ TypeVariableType *ConstraintSystem::openGenericParameter(
   auto *paramLocator = getConstraintLocator(
       locator.withPathElement(LocatorPathElt::GenericParameter(parameter)));
 
-  unsigned options = (TVO_PrefersSubtypeBinding |
-                      TVO_CanBindToHole);
+  unsigned options = TVO_PrefersSubtypeBinding;
+
   if (parameter->isParameterPack())
     options |= TVO_CanBindToPack;
 
+  if (shouldAttemptFixes())
+    options |= TVO_CanBindToHole;
+
   auto typeVar = createTypeVariable(paramLocator, options);
   auto result = replacements.insert(std::make_pair(
       cast<GenericTypeParamType>(parameter->getCanonicalType()), typeVar));