[lit][NFC] Cleanup lit worker process handling

Move code that is executed on worker process to separate file. This
makes the use of the pickled arguments stored in global variables in the
worker a bit clearer. (Still not pretty though.)

Extract handling of parallelism groups to it's own function.

Use BoundedSemaphore instead of Semaphore. BoundedSemaphore raises for
unmatched release() calls.

Cleanup imports.

Differential Revision: https://reviews.llvm.org/D58196

llvm-svn: 354187