Extract Interface/Pull Up: Warn about private members with usages in the original class

#KT-15606 Fixed

idea/testData/refactoring/extractInterface/privateMemberWithUsages.kt

@@ -0,0 +1,8 @@
+// NAME: I
+// SIBLING:
+class <caret>Foo {
+    // INFO: {checked: "true"}
+    private fun privateFun() = 0
+
+    fun refer() = privateFun()
+}