typechecker: unbound name errors in partial/decl mode

Summary: Assuming the existence of ##<?php## code is exceptionally
 useful when starting migration to hack and at the midpoint of the
 migration. Near the end though, it becomes a great burden. To remedy
 that, make the php assumption subject to the whims of ##hh_config##,
 with the default behavior remaining "assuming all-powerful <?php
 implementation for unknown name" but ##assume_php = false## making it
 so that ##Unbound name## errors are thrown for unknown class names.

 Note that, if a codebase has ##assume_php = false## set from the very
 beginning, automated hackification (at least to ##<?hh // decl##) may
 end up done in terms of depth (converting from the root node of class
 hierarchies) rather than breadth (converting from the leaf nodes).

Reviewed By: @int3

Differential Revision: D1753006

Signature: t1:1753006:1419994398:9b6e636e5c1be1f240a31344af5bcd2444b0724e