pfff: auto generate constant builtins from HPHP

Summary:
I used to skip the HPHP constants.idl.json because of limitations
in the third party json parser but doing so leads to
some false positives in cmf about new constants.
This diff fixes that.

Test Plan: make test

Reviewers: pieter, jiaol, amenghra, julienv

Reviewed By: julienv

CC: benjaminstrahs, jdelong, erling

Differential Revision: https://phabricator.fb.com/D888673
* h_program-lang/graph_code.mli: comment
* data/php_stdlib/Makefile: rf
* data/php_stdlib/builtins_constants.idl.php: more
* data/php_stdlib/reflection/reflection.php: