Declare tokenCache on AASTNode

Summary:
See discussion in T10462. Because this class extends Phobject, calling $this->undeclaredProperty = $value; throws.

Declare $tokenCache.

Test Plan: Added debugging statements, saw change cause cache hits to occur.

Reviewers: joshuaspence, chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15358