check @required attribute in codegraph

Summary:
Implemented a check on required xhp field.
         1. modify ast_php_simple to save info about @required token
         2. add Required in database_code, and save it into graph_code
         3. add check_required_field in check_class_php

Test Plan:
have some regression test, working on unit test.
     currently I can test with:
     cd tests/php/codegraph/
     make
     ~/pfff/scheck -with_graph_code save_code.marshall xhp_use.php

Reviewers: pad, pieter, mathieubaudet

Reviewed By: pad

CC: platform-diffs@lists

Differential Revision: https://phabricator.fb.com/D916465