Fix XHPAST to detect use of undeclared variables in catch
Summary:
Currently, when code has a block like:
} catch (Exception $ex) {
...we attempt to mark "$ex" as declared. However, we incorrectly mark every variable used anywhere in the block as declared. This means we'll never raise this warning in a catch block.
Instead only mark the caught exception as declared.
Test Plan: Added a failing unit test and made it pass.
Reviewers: joshuaspence, btrahan
Reviewed By: btrahan
Subscribers: lpriestley, epriestley
Differential Revision: https://secure.phabricator.com/D9239