Verify classes used in typehints

Summary: Also support `SomeInterface::CONSTANT`.

Test Plan:
  interface I {
    const A = 1;
  }
  I::A;

  function f(stdClass $a, array $b, Iterator $c) {
  }

Linted Phabricator.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6135