[CodeCompletion] Fix call-arg completion when we remove all the overloads we found

We have code for pruning unlikely overloads, but when it pruned all
overloads it was treating that as if there was nothing to complete.
Instead, fallback to postfix-expr-begin.

We should also figure out why we're not getting any viable types here,
but we need to handle failure gracefully.

rdar://problem/24356118

lib/IDE/CodeCompletion.cpp

@@ -3505,8 +3505,10 @@ public:
       if (PossibleTypes.empty() &&
           !typeCheckUnresolvedExpr(DC, CallE->getArg(), CallE, PossibleTypes))
         return false;
-      if (RemoveUnlikelyOverloads)
+      if (RemoveUnlikelyOverloads) {
         removeUnlikelyOverloads(PossibleTypes, TupleEleTypesBeforeTarget, &DC);
+        return !PossibleTypes.empty();
+      }
     } else if (CallE->getArg()->getKind() == ExprKind::Paren) {
       Position = 0;
       HasName = false;