Do not generate toString and other methods if they are inherited.

compiler/testData/codegen/boxWithStdlib/dataClasses/tostring/alreadyInherited.kt

@@ -0,0 +1,10 @@
+trait SuperTrait {
+    public fun toString(): String = "!"
+}
+
+data class A(val x: Int): SuperTrait {
+}
+
+fun box(): String {
+  return if (A(0).toString() == "!") "OK" else "fail"
+}