add xhp attr inherit in ast_simple

Summary: To fix possible false positive in codegraph, we have to include xhp attribute's inheritence in ast_simple.

Test Plan:
pfff -dump_php_simple xhp.php
make test

Reviewers: pad, pieter, mathieubaudet

CC: platform-diffs@lists

Differential Revision: https://phabricator.fb.com/D859492