It's easy to introduce bug by adding a new expression and not defining
its ListAssignment RHS kind. Add all expression types explicitly and
remove default case so that it becomes compile time error if the list is
not updated.
This change is mostly for FB internal organizational reasons.
Building is not effected beyond the fact that the target now
lands in hphp/hhvm/hhvm rather than src/hhvm/hhvm.
jan