[TestFormatters, TestDataFormatterSynthVal] Mark as xfail.

Summary:
This is to get the bots to go green while a robust solution to fix these
tests is worked out.

Reviewers: chaoren, tberghammer, clayborg

Reviewed By: tberghammer, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8947

llvm-svn: 234625