[PATCH] [Lit] Use multiprocessing instead of threading

[Daniel Dunbar]

  Hi Takumi,

  I just pushed an implementation of multiprocessing in lit, currently
  available under the --use-processes command line flag.

  If you have a moment, could you rerun your previous timings (or similar) to
  confirm that the version I pushed shows the same improvements you
  originally reported?

  Also, I haven't yet tested this feature on Windows, so I would love to hear
  if it works and what the performance impact is. Fundamentally it has the
  same pickling issues as before so if you previously found that to be a
  bottleneck then it may continue to be a problem.

   - Daniel

http://llvm-reviews.chandlerc.com/D134