Add support for using class constants as shape field names.

Summary: This makes pfff support class constants as shape field names. (Actually, it now allows *anything* as a shape field name, but I don't think this is a problem since other tools will catch that.)

Test Plan: Added a test

Reviewers: pad, jwatzman

Differential Revision: https://phabricator.fb.com/D1476673

Tasks: 4513262