[sanitizer-coverage] Mark the guard and 8-bit counter arrays as used

In r311742 we marked the PCs array as used so it wouldn't be dead
stripped, but left the guard and 8-bit counters arrays alone since
these are referenced by the coverage instrumentation. This doesn't
quite work if we want the indices of the PCs array to match the other
arrays though, since elements can still end up being dead and
disappear.

Instead, we mark all three of these arrays as used so that they'll be
consistent with one another.

llvm-svn: 311959