Issue #2313479 by tim.plunkett: Add FormState::setResponse().

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

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

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

core/modules/locale/src/Form/ExportForm.php

...tests/modules/form_test/src/Form/FormTestCheckboxForm.php

...dules/form_test/src/Form/FormTestCheckboxesRadiosForm.php

...modules/form_test/src/Form/FormTestCheckboxesZeroForm.php

...em/tests/modules/form_test/src/Form/FormTestColorForm.php

...dules/form_test/src/Form/FormTestDisabledElementsForm.php

...em/tests/modules/form_test/src/Form/FormTestEmailForm.php

...modules/form_test/src/Form/FormTestLanguageSelectForm.php

...em/tests/modules/form_test/src/Form/FormTestRangeForm.php

...m/tests/modules/form_test/src/Form/FormTestSelectForm.php

...stem/tests/modules/form_test/src/Form/FormTestUrlForm.php

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