Make lit stop writing pyc files.

Make lit stop writing pyc files.

Many svn-based buildbots seem to be getting stuck continually
in tree conflicts due to the output of pyc files.  I'm disabling
these as a temporary measure in an attempt to get everything
stable again.

I'll try to remove this code once I understand the problem
better.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313698 91177308-0d34-0410-b5e6-96231b3b80d8