Simplify AASTNodeList::selectDescendantsOfType

Summary: Rewrite AASTNodeList::selectDescendantsOfType to just call AASTNodeList::selectDescendantsOfTypes, minimizing code duplication.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12287