Add test case for https://bugs.swift.org/browse/SR-7771

Author: slavapestov

test/SILGen/default_arguments.swift

@@ -376,4 +376,10 @@ func tupleDefaultArg(x: (Int, Int) = (1, 2)) {}
 // CHECK: return
 func callTupleDefaultArg() {
   tupleDefaultArg()
-}
+}
+
+// FIXME: Should this be banned?
+func stupidGames(x: Int = 3) -> Int {
+  return x
+}
+stupidGames(x:)()