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