Move the access logic in FormBuilder::handleInputElement() to its own protected method

Problem/Motivation

FormBuilder::handleInputElement() contains a long condition that checks if #access is a boolean or a valid access result object:

$process_input = empty($element['#disabled']) &&
      !in_array($element['#type'], ['item', 'value'], TRUE) &&
      (
        ($form_state->isProgrammed() && $form_state->isBypassingProgrammedAccessChecks()) ||
        ($form_state->isProcessingInput() && (!isset($element['#access']) || (($element['#access'] instanceof AccessResultInterface && $element['#access']->isAllowed()) || ($element['#access'] === TRUE))))
      );

You can check here

Moving this condition to a separate method would make it easier to maintain, document and reuse.

Steps to reproduce

1. On a Drupal up-to-date and Clean installation, create a simple form with: drush gen form
2. Follow those prompts, and make sure your new form use a textfield '#type'
3. Debug the $process_input var after submit any value
4. The result should be the same before and after the changes being applyied

Proposed resolution

Create a new private method that returns a boolean.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet