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

#KT-11799 Fixed

idea/testData/quickfix/createFromUsage/createFunction/call/extensionFunOnTrait.kt.after

@@ -6,6 +6,6 @@ fun test(t: T) {
     val b: Boolean = t.foo("1", 2)
 }
 
-fun T.foo(s: String, i: Int): Boolean {
+private fun T.foo(s: String, i: Int): Boolean {
     TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
 }