Description:
------------
The `Stringable` interface available as of 8.0.0, which can be explicitly or implicitly available on a class that implements `__toString()` does not seem to acknowledge that if `__toString()` comes via a trait, then the implicit implementation of `Stringable` does not work

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

trait A {
    public function __toString(): string {
        return 'ok';
    }
}

class B {
    use A;
}

class C {
    public function __toString(): string {
        return 'ok';
    }
}

$b = new B();

var_dump($b instanceof Stringable);

$c = new C();

var_dump($c instanceof Stringable);

Expected result:
----------------
PHP7:
bool(false)
bool(false)

PHP8:
bool(true)
bool(true)

Actual result:
--------------
PHP7:
bool(false)
bool(false)

PHP8:
bool(false)
bool(true)