Support autocreation for views selection

Problem/Motivation

When autocreation for a views selection is used, like in #2800875: Autocreate for Entity Reference selection handlers that extend ViewsSelection, it doesn't work with select2.

Proposed resolution

Copy the changes from Drupal\Core\Field\Plugin\Field\FieldWidget\EntityReferenceAutocompleteWidget::getAutocreateBundle to Select2EntityReferenceWidget::getAutocreateBundle().