[5.4] Fix JoinClause's whereIn when using a subquery

[rtheunissen]

Description:

You should be able to pass an existing Builder to whereIn of a JoinClause, but this doesn't work because the if ($values instanceof static) doesn't catch the expected case when you pass a Builder because it's not an instance of JoinClause.

TypeError: Argument 1 passed to Illuminate\Database\Grammar::parameterize() must be of the type array, object given

Steps To Reproduce:

/** @var Illuminate\Database\Query $query */
$query = $this->getFieldQuery(); 

Account::join('other_table', function (JoinClause $join) use ($query) {
    return $join->on('other_table.field', '=', 'accounts.field')
                ->whereIn('other_table.field', $query);
});

[rtheunissen]

Thoughts on this @taylorotwell? We currently can't use subqueries in joins resulting in more queries than we'd like to be making. Should we use a fork in the meantime (rather than modified vendor files) or can we look at releasing this as a patch on 5.4?