Update xhpast linter rules for new function AST format
Summary: See D15678. A node containing a return type hint is added right before the statement body node. This updates all relevant linter rules to now retrieve the body from the correct index.
Test Plan: Ran arc unit --everything, inspected every single message to make sure all xhpast test cases succeeded. Also grepped for getChildByIndex(5 and getChildOfType(5.
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D15814