Add test for external call inside operator function

test/libsolidity/semanticTests/operators/custom/external_call_inside_operator_function.sol

@@ -0,0 +1,24 @@
+type Int is int16;
+
+using {add as +} for Int;
+
+
+function add(Int, Int) returns (Int) {
+    B b = new B();
+    return b.f();
+}
+
+contract B {
+    function f() pure external returns (Int) {
+        return Int.wrap(3);
+    }
+}
+
+contract C {
+    function test() public returns (Int) {
+        return Int.wrap(0) + Int.wrap(0);
+    }
+}
+
+// ----
+// test() -> 3