Description:
------------
__PHP_Incomplete_Class should be final and not cloneable.
There is no meaning in letting the possibility to extend it.

This change would be a BC break but it shouldn't impact anyone.

Test script:
---------------
<?php

$c = new class('bar') extends __PHP_Incomplete_Class {
    public $foo;
    
    public function __construct($foo) {
        $this->foo = $foo;
    }
};

var_dump($c);

Expected result:
----------------
Fatal error: Class class@anonymous may not inherit from final class (__PHP_Incomplete_Class) in /in/Vl5Ep on line 3

Actual result:
--------------
Notice: class@anonymous::__construct(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "unknown" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide an autoloader to load the class definition in /in/Vl5Ep on line 7
object(class@anonymous)#1 (1) {
  ["foo"]=>
  NULL
}