Extract Superclass: Report private members used in abstracted members

#KT-16284 Fixed

idea/testData/refactoring/extractInterface/abstractedMemberUsingPrivate.kt

@@ -0,0 +1,9 @@
+// NAME: I
+
+// SIBLING:
+class <caret>PrivateRef {
+    // INFO: {checked: "true", toAbstract: "false"}
+    private fun privateFun() = 0
+    // INFO: {checked: "true", toAbstract: "true"}
+    fun refer() = privateFun()
+}