Mark test_watchpoint_multiple_threads_with_dwarf as XFAIL on Linux/clang.

See http://llvm.org/bugs/show_bug.cgi?id=20233 for details.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@212491 91177308-0d34-0410-b5e6-96231b3b80d8