[pfff] parse `+` and `-` in type parameters sing `git commit -m 'One line title' && arc diff`.

Summary:
Looks like we added this without updating the PFFF parser. This
adds parsing support but doesn't include the `+` / `-` in the
AST later.

Test Plan:
  $ cat test.php
  <?hh // strict
  // Copyright 2004-present Facebook. All Rights Reserved.

  class Derp<-SomeClass, +SomeClass> {

  }

  $ ./pfff -dump_php ./test.php

  [ClassDef(
   {c_type=ClassRegular(i_1); c_name=Name(("Derp", i_2));
    c_tparams=Some((i_3,
                    [Left(TParam(Name(("SomeClass", i_4)))); Right(i_5);
                     Left(TParam(Name(("SomeClass", i_6))))],
                    i_7));
    c_extends=None; c_implements=None; c_attrs=None; c_body=(i_8, [], i_9);
    c_enum_type=None; }); FinalDef(i_10)]

Reviewers: pad, julienv, andrewparoski, gigz

Reviewed By: gigz

Subscribers: gigz, dominik

Differential Revision: https://phabricator.fb.com/D1638340