[5.6] Ignore non-where bindings in nested where() constraints

[staudenmeir]

Nested where() constraints merge all bindings from the given query, even the ones from other query parts.

This breaks queries that reference a relationship with WHERE bindings (e.g. MorphMany) in $withCount and use a nested where() constraint:

class ParentModel extends Model
{
    protected $table = 'parent';

    protected $withCount = ['children'];

    public function children()
    {
        return $this->morphMany(ChildModel::class, 'morphable');
    }
}

class ChildModel extends Model
{
    protected $table = 'child';
}

$query = ParentModel::where(function($q) {
    $q->where(1, 1);
});

The query doesn't work as expected because an extra binding is added:

dd($query->getBindings());
// expected: ['App\ParentModel', 1];
// actual: ['App\ParentModel', 'App\ParentModel', 1]

As a result 'App\ParentModel' is used for the second placeholder and 1 is ignored.

This is caused by Model::newQueryWithoutScopes() adding $withCount to every query.
So the query received by Builder::where() contains the $withCount bindings.

This PR solves the problem by ignoring non-where bindings when the nested query is merged.

Fixes #23957.

[morloderex]

tests for this?

[staudenmeir]

->addBinding('ignore', 'select') tests that non-where bindings are ignored. Before this change 'ignore' would have been added to the bindings and the test would have failed.

[taylorotwell]

Please re-submit with an entirely new test that shows the corrected behavior and would have been broken before. Try not to modify existing tests.