Tag changes made with HHVM

If HHVM introduces parsing errors or irregularities, we'd want to be able to
identify revisions that were potentially affected, so we can fix them. So add
an 'HHVM' change tag to revisions made with HHVM.

Change-Id: I4958aff234df065480e350f3e4ac02e032964040
(cherry picked from commit 82406936778e10e5ebf85b3d2b3efce1181ac3eb)