This is a followup to https://reviews.llvm.org/D129814

That was causing hit counts to be double-counted on x86_64 Linux.
It looks like StopInfoWatchpoint::ShouldStopSynchronous gets called
twice for a give stop on Linux (not on Darwin).  I had taken out the
"have I been called already" check when I reworked this part of the
code because it didn't seem necessary.  Putting that back in because
it looks like it is on some systems.