edit a test php file to test field
add: check field in xhp

Summary: I added a xhp_field in ast_php_simple. The ast now can keep track of field in a xhp class, and do checks.

Test Plan: on test/php/codegraph/xhp.php and xhp_use.php

Reviewers: pad

CC: platform-diffs@lists

Differential Revision: https://phabricator.fb.com/D847305

Task ID: 2489777