Disallow overriding operators for enums

test/libsolidity/syntaxTests/operators/custom/operator_bound_to_enum.sol

@@ -0,0 +1,13 @@
+using {add as +} for E;
+
+enum E {
+    E1,
+    E2
+}
+
+function add(E, E) pure returns (E) {
+    return E.E1;
+}
+
+// ----
+// TypeError 9921: (0-23): The "using" directive cannot be used to attach functions to the enum type.