A third attempt to mark TestRdar12408181.py as skipped

Due to an unfortunate difference between the open source test harness
and our internal harness, applying two @skip... decorators to this test
works in the internal build but not in the open source build.

I've tried another approach to skipping this test and tested it out with
the open source harness. Hopefully this sticks!

rdar://36417163

llvm-svn: 322756