[pfff] Add class constant typehints to the AST

Summary: Typehints for class constats seem to be parsed but not saved in the AST. Not sure if I did this right; mostly copied the code for class variables.

Test Plan: make clean / make all works correctly; I'm successfully using the class constant typehints in the Codex script (which uses pfff)

Reviewers: pad

Reviewed By: pad

Differential Revision: https://phabricator.fb.com/D1422739

Tasks: 4392286
[codex] PHP parser