Provide additional Arcanist PHP 8.1 fixes
Summary: Ref T13588. I pointed my local php at PHP 8.1 and this is what I've hit so far; all these cases seem very unlikely to have any subtle behavior.
Test Plan: Ran various arc workflows under PHP 8.1.
Maniphest Tasks: T13588
Differential Revision: https://secure.phabricator.com/D21742