Create from Usage: Make extension functions/properties 'private' by default

#KT-11799 Fixed

idea/testData/quickfix/createFromUsage/createFunction/get/createGetFromUsage2.kt.after

@@ -5,6 +5,6 @@ fun x (y: Any) {
     val z: Any = y[""]
 }
 
-operator fun Any.get(s: String): Any {
+private operator fun Any.get(s: String): Any {
     TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
 }