3 files declare their use of OptGroup

EntityReferenceItem.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
OptGroupTest.php in core/tests/Drupal/Tests/Core/Form/OptGroupTest.php
OptionsWidgetBase.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/OptionsWidgetBase.php

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