Find classes used in call_user_func()

Summary:
Also fix false positive for call_user_func('C::f').
Also fix false positive for call_user_func("$f").
Also centralize handling of magic classes.

Test Plan:
  call_user_func("$f");
  call_user_func('C::f');
  call_user_func(array('C', 'f'));
  call_user_func(array(1 => 'f', 0 => 'C'));

Linted Phabricator.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6137