[lit] Add a single process mode.

[lit] Add a single process mode.

This is helpful for debugging test failures since it removes
the multiprocessing pool from the picture.  This will obviously
slow down the test suite by a few orders of magnitude, so it
should only be used for debugging specific failures.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313460 91177308-0d34-0410-b5e6-96231b3b80d8