[code-completion] Handle func-reference syntax for optional expected type

The underlying type-check was correct, but I forgot to consider it in
the outer code, and embarassingly never tested this case.

rdar://problem/28435922

lib/IDE/CodeCompletion.cpp

@@ -1652,7 +1652,9 @@ private:
       return true;
     bool isImplicitlyCurriedIM = isImplicitlyCurriedInstanceMethod(D);
     for (auto expectedType : ExpectedTypes) {
-      if (expectedType && expectedType->is<AnyFunctionType>() &&
+      if (expectedType &&
+          expectedType->lookThroughAllAnyOptionalTypes()
+              ->is<AnyFunctionType>() &&
           calculateTypeRelationForDecl(D, expectedType, isImplicitlyCurriedIM,
                                        /*UseFuncResult=*/false) >=
               CodeCompletionResult::ExpectedTypeRelation::Convertible) {