Add a Line Item Views filter to restrict results to non-orphaned line items

Since Drupal Commerce depends on a bi-directional reference between orders and line items, it is possible for line items to exist that either don't reference an order or that reference an order that does not reference it in turn. This is problematic when a site builder wants to create a View of line items, because it includes items in the result set that wouldn't be acknowledged anywhere else. We need a filter that can restrict the result set to only include non-orphaned line items.