pfff: allow traits to implement interfaces in the parser

Test Plan:
$ make test
$ ~/pfff/pfff_test -dump_php_simple traits_interface.php
[ClassDef(
   {c_name="Iface"; c_kind=Interface; c_extends=None; c_implements=[];
    c_attrs=[]; c_constants=[]; c_xhp_attr_inherit=[]; c_xhp_fields=[
    ]; c_variables=[]; c_body=[]; });
 ClassDef(
   {c_name="T"; c_kind=Trait; c_extends=None; c_implements=[Hint(["Iface"])];
    c_attrs=[]; c_constants=[]; c_xhp_attr_inherit=[]; c_xhp_fields=[
    ]; c_variables=[]; c_body=[]; });
 ClassDef(
   {c_name="C"; c_kind=ClassRegular; c_extends=None; c_implements=[];
    c_attrs=[]; c_constants=[]; c_xhp_attr_inherit=[]; c_xhp_fields=[
    ]; c_variables=[]; c_body=[]; });
 FuncDef(
   {f_name="fn"; f_kind=Function;
    f_params=[{p_name="$i"; p_type=Some(Hint(["Iface"])); p_attrs=[];
               p_ref=false; p_default=None; }];
    f_ref=false; f_return_type=None; m_modifiers=[]; l_uses=[]; f_attrs=[
    ]; f_body=[]; })]

Reviewers: ide, amenghra, julienv, pieter, jiaol

Reviewed By: amenghra

Differential Revision: https://phabricator.fb.com/D889900

Task ID: 1962369
Merge branch 'master' of github.com:facebook/pfff