[5.8] Fix self-referencing MorphOneOrMany existence queries

[staudenmeir]

Self-referencing existence queries of MorphOneOrMany relationships are incorrect:

class Comment extends Model
{
    public function replies()
    {
        return $this->morphMany(self::class, 'commentable');
    }
}

Comment::has('replies')->get();

# expected
select * from `comments` where exists (
  select * from `comments` as `laravel_reserved_0`
  where `comments`.`id` = `laravel_reserved_0`.`commentable_id`
  and `laravel_reserved_0`.`commentable_type` = ?
)

# actual
select * from `comments` where exists (
  select * from `comments` as `laravel_reserved_0`
  where `comments`.`id` = `laravel_reserved_0`.`commentable_id`
  and `comments`.`commentable_type` = ?
)      ^^^^^^^^