[DX] Provide a version of check_plain() that does not escape (for RSS feeds)

I've been suffering from the same issue as these people: * https://www.drupal.org/project/views_rss/issues/1674134 * https://www.drupal.org/project/views/issues/1189550 * https://stackoverflow.com/questions/14229641/rss-feeds-view-showing-enco...

I have resorted to both removing the check_plain() calls that are added in template_preprocess_views_view_row_rss and replaced them with decode_entities() to get my RSS feeds to be clean. But I wonder about the security implications of doing so.

We should provide a sanitization function that does not escape characters, something like check_plain_rss() maybe? What are our options currently?