Streamline test case by avoiding a temporary file and piping llc output straight to llvm-dwarfdump

We still do temporary files in many cases, just updating this particular
one because I was debugging it and made this change while doing so.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209601 91177308-0d34-0410-b5e6-96231b3b80d8