Description:
------------
Consider this class:

class C
{
    public function m(): stdClass|static
    {
    }
}

ReflectionMethod::getReturnType() returns for C::m() a ReflectionUnionType that contains only a ReflectionNamedType object for "stdClass" but no information about "static".

This blocks PHPUnit's support for test doubles of objects with methods that declare a return type with a union that includes "static" (https://github.com/sebastianbergmann/phpunit/issues/4480, https://github.com/sebastianbergmann/type/issues/14).

Test script:
---------------
<?php declare(strict_types=1);
class C
{
    public function a(): self
    {
    }

    public function b(): stdClass|self
    {
    }

    public function c(): static
    {
    }

    public function d(): stdClass|static
    {
    }
}

foreach ((new ReflectionClass(C::class))->getMethods() as $method) {
    print $method->getDeclaringClass()->getName() . '::' . $method->getName() . '()' . PHP_EOL;
    print '    $method->getReturnType() returns ' . get_class($method->getReturnType()) . PHP_EOL;
    print '    $method->getReturnType()->__toString() returns ' . $method->getReturnType() . PHP_EOL;

    if ($method->getReturnType() instanceof ReflectionUnionType) {
        print '    $method->getReturnType()->getTypes() returns an array with ' . count($method->getReturnType()->getTypes()) . ' element(s)' . PHP_EOL;

        print '    type(s) in union: ';
        
        $types = [];

        foreach ($method->getReturnType()->getTypes() as $type) {
            $types[] = get_class($type);
        }
        
        print join(', ', $types) . PHP_EOL;
    }

    print PHP_EOL;
}


Expected result:
----------------
C::a()
    $method->getReturnType() returns ReflectionNamedType
    $method->getReturnType()->__toString() returns self

C::b()
    $method->getReturnType() returns ReflectionUnionType
    $method->getReturnType()->__toString() returns stdClass|self
    $method->getReturnType()->getTypes() returns an array with 2 element(s)
    type(s) in union: ReflectionNamedType, ReflectionNamedType

C::c()
    $method->getReturnType() returns ReflectionNamedType
    $method->getReturnType()->__toString() returns static

C::d()
    $method->getReturnType() returns ReflectionUnionType
    $method->getReturnType()->__toString() returns stdClass|static
    $method->getReturnType()->getTypes() returns an array with 1 element(s)
    type(s) in union: ReflectionNamedType, SOMETHING FOR STATIC


Actual result:
--------------
C::a()
    $method->getReturnType() returns ReflectionNamedType
    $method->getReturnType()->__toString() returns self

C::b()
    $method->getReturnType() returns ReflectionUnionType
    $method->getReturnType()->__toString() returns stdClass|self
    $method->getReturnType()->getTypes() returns an array with 2 element(s)
    type(s) in union: ReflectionNamedType, ReflectionNamedType

C::c()
    $method->getReturnType() returns ReflectionNamedType
    $method->getReturnType()->__toString() returns static

C::d()
    $method->getReturnType() returns ReflectionUnionType
    $method->getReturnType()->__toString() returns stdClass|static
    $method->getReturnType()->getTypes() returns an array with 1 element(s)
    type(s) in union: ReflectionNamedType