llvm-lit JUnit XML output

Hi,

To integrate LLVM and our LLVM-based tools into our Jenkins CI system, I've patched llvm-lit to be able to produce JUnit XML output in addition to its normal output. We're currently applying a patch to the checkout, but I wondered if this would be of use to anyone else. If so, I'd be happy to submit the patch for review.

David

Many current CI systems parse JUnit-style output. It is buildbot that is deficient here. I'd be for accepting such a change, then we can get buildbot to move forward.