llvm-profdata: Avoid writing to /dev/null in tests

We fseek on our output file in llvm-profdata, which errors on some
systems. Avoid getting into the situation by writing to /dev/null

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206670 91177308-0d34-0410-b5e6-96231b3b80d8