llvm-cov: Move XFAIL after the body of the test

llvm-cov tests are sensitive to line number changes, so putting this
at the end will limit churn when we fix the XFAIL.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204871 91177308-0d34-0410-b5e6-96231b3b80d8