Don't throw PhutilMissingSymbolException from within method_exists and property_exists calls
Summary: Ref T1116. Fixes T9954. It seems that method_exists and propery_exists can trigger PHP autoloaders as well.
Test Plan: Fixed the issue described in T9954.
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D14860