Remove some incorrect restrictions in the grammar of constant expressions.

Summary: Right now, constant expressions can only contain unary negations of scalar values, when php 5.6 (and common sense) allows negations of any static expressions. Also, < and > require their second argument to be a static_scalar also, which is also silly and doesn't match php 5.6. (In fact, the whole notion of static_scalar doesn't seem to be useful.) Also add support for ** in constant exprs.

Reviewed By: @elgenie

Differential Revision: D1457346