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