[pfff] handle ($o->fn)(...) special syntax

Summary:
There was an ambiguity between $o->fn() and ($o->fn)() in ast_php_simple.
The parenthesis in this case have a meaning to disambiguate those
two cases.

Test Plan:
$ ~/pfff/pfff_test -dump_php_simple field_call.php
...
 Expr(
   Call(Id(["var_dump"]),
     [Call(Id(["call_user_func"]), [Obj_get(Var("$o"), Id(["fn"]))])]));
 Expr(
   Call(Id(["var_dump"]),
     [Call(Id(["call_user_func"]), [Obj_get(Var("$o"), Id(["fn"]))])]))]

Reviewers: amenghra, julienv

Reviewed By: amenghra

CC: andrewparoski, bryano

Differential Revision: https://phabricator.fb.com/D1030430
* data/php_stdlib/builtins_constants.idl.php:
* tests/php/codegraph/class.php: unit test for ($o->fn)() in codegraph