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()
+}

idea/testData/refactoring/extractInterface/abstractedMemberUsingPrivate.kt.conflicts

@@ -0,0 +1 @@
+Function 'fun refer(): Int' uses function 'fun privateFun(): Int' which won't be accessible from the subclass.

idea/testData/refactoring/extractInterface/privateMemberWithUsages.kt.after

@@ -0,0 +1,11 @@
+interface I {
+    // INFO: {checked: "true"}
+    fun privateFun() = 0
+}
+
+// NAME: I
+// SIBLING:
+class Foo : I {
+
+    fun refer() = privateFun()
+}

idea/testData/refactoring/extractInterface/privateMemberWithUsages.kt.conflicts

@@ -1 +0,0 @@
-Function 'fun privateFun(): Int' can't be moved to the interface because it's private and has usages in the original class