Issue #2209977 by tim.plunkett: Move form validation logic out of FormBuilder into a new class.

core/core.services.yml

core/includes/form.inc

core/lib/Drupal/Core/Form/FormBuilder.php

core/lib/Drupal/Core/Form/FormBuilderInterface.php

core/lib/Drupal/Core/Form/FormValidator.php

core/lib/Drupal/Core/Form/FormValidatorInterface.php

core/lib/Drupal/Core/Form/OptGroup.php

...upal/entity_reference/ConfigurableEntityReferenceItem.php

...ib/Drupal/options/Plugin/Field/FieldType/ListItemBase.php

...nomy/Plugin/Field/FieldType/TaxonomyTermReferenceItem.php

core/tests/Drupal/Tests/Core/Form/FormBuilderTest.php

core/tests/Drupal/Tests/Core/Form/FormTestBase.php

core/tests/Drupal/Tests/Core/Form/FormValidationTest.php

core/tests/Drupal/Tests/Core/Form/FormValidatorTest.php