Handle not unique operators for user-defined type.

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

@@ -0,0 +1,17 @@
+type Int is uint128;
+
+using {add as +, sub as +} for Int;
+
+function add(Int, Int) returns (Int) {
+    return Int.wrap(0);
+}
+
+function sub(Int, Int) returns (Int) {
+    return Int.wrap(0);
+}
+
+function test() {
+    Int.wrap(0) + Int.wrap(1);
+}
+// ----
+// TypeError 2271: (213-238): Operator + not compatible with types Int and Int. A user-defined operator not unique.