DRD seems to prefer those annotations to
DRD_IGNORE_VAR/DRD_STOP_IGNORING_VAR.

Helgrind is stupid, and VALGRIND_HG_{DIS,EN}ABLE_CHECKING pairs
bizarrely "reclaim" the memory location for the current thread when
re-enabling checking.  So they probably aren't completely suitable for
our purposes, but Helgrind provides no facility for doing what we want
because it just wants to watch the world burn.  So we add a couple of
Helgrind suppressions in addition.

Also, added a cute RAII wrapper for reading a variable in a racy way.
Writing can be done the same way if we only want to appease DRD, but
because Helgrind does things on a memory location-basis and not a
thread-basis, things get a little tricky.