Adjust parser to complain when $this is reassigned on method declaration

Summary: Aims to fix #3940. I'm in doubt about the distinction between PHP errors HipHop error messages though.
Closes https://github.com/facebook/hhvm/pull/4095

Reviewed By: @ptarjan

Differential Revision: D1646085

Signature: t1:1646085:1414650363:07cdd19d0fda1588b3ac1a75fbf6cf637d8db556