Generate nullability annotations on this receiver in DefaultImpls. Don't generate nullability annotations in private methods

#KT-36969 Fixed

compiler/testData/codegen/bytecodeListing/extension.kt

@@ -0,0 +1,12 @@
+
+fun String.test() {}
+
+private fun String.testPrivate() {}
+
+var String.prop
+    get() = "123"
+    set(value) {}
+
+private var String.propPrivate
+    get() = "123"
+    set(value) {}