[lit] Speculatively fix PR24554 by manually closing the process handle

My theory is that somehow Python's refcounting and GC strategy isn't
closing the subprocess handle in a timely fashion.  This accesses the
private '_handle' field of the Popen object, but I see no other way to
do this. If this doesn't address the problem on the sanitizer-windows
buildbot, we can revert this change. If it does, then let's keep the
hack.

llvm-svn: 245946