Add a comment to tests that XPASS on FreeBSD

Perhaps these should be @expectedFailureDarwin instead of
@unittest2.expectedFailure (applying to all hosts); I'm not aware of
the details in the rdars.

Just add a comment for now, for the benefit of anyone investigating
FreeBSD test issues in the future.

rdar://9980907
rdar://15367233

llvm-svn: 206760