Description:
------------
The in php 8 added "ReflectionUnionType" class should extend the ReflectionType class according to the RFC proposal (https://wiki.php.net/rfc/union_types_v2#reflection), but the inherited "isBuiltin" method (https://www.php.net/manual/de/reflectiontype.isbuiltin.php) does not exist.

Example script: https://3v4l.org/5BDRn
How it looks like without union type: https://3v4l.org/TGkX2

Test script:
---------------
<?php
class SomeClass
{
    protected string|stdClass $a;
}

$unionType = (new \ReflectionClass(SomeClass::class))->getProperty('a')->getType();
var_dump($unionType->isBuiltin());

Expected result:
----------------
bool(false)

Actual result:
--------------
Fatal error: Uncaught Error: Call to undefined method ReflectionUnionType::isBuiltin() in /in/5BDRn:8
Stack trace:
#0 {main}
  thrown in /in/5BDRn on line 8

Process exited with code 255.