Drupal 8: Add placeholder attribute to an input form element

Adding placeholder text to a Drupal 8 form element that is the label of the form can be done in your .theme file with the hook template_preprocess_input().

/**
 * Implements template_preprocess_input().
 */
function MYTHEME_preprocess_input(&$variables) {
  // Set a placeholder for all search form elements.
  if ($variables['attributes']['type'] == 'search') {
    $variables['attributes']['placeholder'] = $variables['element']['#title']; 
  }
}

The code above will add the #title of the form field as a placeholder attribute to any field of type 'search'.