RFC: [pfff] Fix issue with lexing XHP identifiers

Summary:
This fixes pfff's lexer to match the behavior of github.com/facebook/xhp,
HHVM, and the Hack typechecker.

Test Plan: ?

Reviewers: andrewparoski

Reviewed By: andrewparoski

Differential Revision: https://phabricator.fb.com/D1745109

Signature: t1:1745109:1418805782:6e96e2b4c69cbe383de76b61400dfd35a4fe3193