typechecker add PREG_* constants to hhi

Summary: Predefined preg constants as listed on http://docs.hhvm.com/manual/en/pcre.constants.php

 Closes https://github.com/facebook/hhvm/pull/4281

Reviewed By: @paulbiss

Differential Revision: D1693307

Signature: t1:1693307:1416460125:d69a7740e75fdf672d9de26c7b1cef9b258b9b6e