Return passed value from phlog()

Summary:
It is very handy to add logging just by wrapping value inside expression, e.g.

list($a, $b) = phlog(f());

Test Plan: $a = phlog('a');

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1933