Add test for this fix.

test/fail_compilation/fail13116.d

@@ -0,0 +1,29 @@
+/*
+TEST_OUTPUT:
+---
+fail_compilation/fail13116.d(13): Error: this is not an lvalue
+---
+*/
+struct S
+{
+    ref S notEvil() { return this; } // this should be accepted
+}
+class C
+{
+    ref C evil() { return this; } // this should be rejected
+}
+void main()
+{
+}
+
+/*
+TEST_OUTPUT:
+---
+fail_compilation/fail13116.d(28): Error: super is not an lvalue
+---
+*/
+class Base { }
+class Derived : Base
+{
+    ref Base evil() { return super; } // should be rejected
+}