Handle static scalar in xhp attribute default value context

Test Plan:
cat test.php | ./xhpast
where test.php is:

<?php

class Foo {
  const CST = 1;
}

class :x:foo {
  attribute
    int padding = Foo::CST,
    ;
}

Reviewed By: epriestley
Reviewers: epriestley, marcel
CC: aran, pad, epriestley
Revert Plan:
Tags:

• begin *PUBLIC* platform impact section -

Bugzilla: #

• end platform impact -

Differential Revision: 232