[pfff] parse + - type parameter constraints in combination with `as`

Summary:
Looks like I went a little too easy D1638340; also
[[https://www.facebook.com/groups/hackforhiphop/permalink/761864200528864/|hack group discussion]].

Test Plan:
  ./pfff -dump_php ./test.php
  [ClassDef(
   {c_type=ClassAbstract(i_1, i_2); c_name=Name(("MobileZeroTargeter", i_3));
    c_tparams=Some((i_4,
                    [Left(
                       TParamConstraint(Name(("T", i_5)), i_6,
                         Hint(
                           XName([QI(Name(("IMobileZeroTargetable", i_7)))]),
                           None)))],
                    i_8));
    c_extends=None; c_implements=None; c_attrs=None; c_body=(i_9, [], i_10);
    c_enum_type=None; }); FinalDef(i_11)]
  [pieter@dev1342 ~/pfff] cat test.php
  <?hh
  abstract class MobileZeroTargeter<-T as IMobileZeroTargetable> {

  }

Reviewers: jwatzman, eletuchy

Reviewed By: eletuchy

Subscribers: dominik

Differential Revision: https://phabricator.fb.com/D1659736

Signature: t1:1659736:1415149410:0ca7d109400b0820bf9dd34ef560edf3ac2ffc3a