[5.6] Fix MorphTo eager loading and withoutGlobalScopes()

[staudenmeir]

withoutGlobalScopes() doesn't work when eager loading MorphTo relationships:

class Comment extends Model {
    public function commentable() {
        return $this->morphTo()->withoutGlobalScopes();
    }
}

class Post extends Model {
    use SoftDeletes;
}

Comment::with('commentable')->get();

# expected
select * from "posts" where "posts"."id" in (?, ?)

# actual
select * from "posts" where "posts"."id" in (?, ?) and "posts"."deleted_at" is null

MorphTo uses a temporary query (based on Comment) to build the relationship. At this point, withoutGlobalScopes() can only remove scopes that were registered on Comment. When getResultsByType() finally executes individual queries for the different related models, the global scopes get re-applied.

We can fix it by catching the withoutGlobalScopes() calls and using the $macroBuffer to apply them later to the individual queries.

Fixes #18052 and #21856.