Extract Superclass: Allow extraction to existing file

#KT-15351 Fixed

idea/testData/refactoring/extractInterface/extractToExistingFile.kt

@@ -0,0 +1,6 @@
+// NAME: ISomething
+// TARGET_FILE_NAME: extractToExistingFile.1.kt
+class <caret>Something {
+    // INFO: {checked: "true"}
+    var independent = 1
+}