Mark failing tests in TestDataFormatterObjC on Darwin as XFAIL

See http://llvm.org/bugs/show_bug.cgi?id=20260 for more details.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@212639 91177308-0d34-0410-b5e6-96231b3b80d8