[5.2] fix using onlyTrashed and withTrashed with whereHas

[themsaid]

For Post having many Comment models, Comment is using soft deletion:

Post::has('comments')->get(); 
// Returns a post that has comments where `comments.deleted_at IS NULL`.

Post::whereHas('comments', function($q){
    $q->withTrashed();
})->get();
// Should return posts that has comments even if soft deleted.

Post::whereHas('comments', function($q){
    $q->onlyTrashed();
})->get();
// Should return posts that has all comments soft deleted.

Currently case 2 & 3 aren't working, that's because the soft deletion query scope adds a whereNull condition on every query, this PR manages to remove this condition when needed. Here's a sample query that demonstrates case 2:

select * from `posts` where exists (select * from `comments` where `comments`.`post_id` = `posts`.`id` and `comments`.`deleted_at` is not null and `comments`.`deleted_at` is null)

NOTE I'm not sure if that's the right way to accomplish the task, I tried to fix it with a minimal effect on the current code.

[taylorotwell]

It seems like this is taking us back to really brittle column name comparisons like we used to have on scopes.

[taylorotwell]

I'm curious if @acasar has a different solution or alternative to this because I don't want to go back to comparing column names.

[acasar]

@taylorotwell I think I found a better way to solve this problem: #13396

@themsaid I used your tests, but implemented the logic a bit differently. Can you check if this fixes your issue?

[themsaid]

@acasar I think that would do yes, & it's also implemented in a cleaner way 👍
I'm closing this PR then.