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

Maniphest Tasks: T1116, T9954

Differential Revision: https://secure.phabricator.com/D14860