lit: Issue a note when multiprocessing fails to load

If multiprocessing was requested, detected as available and subsequently failed
to initialize it's worth letting the user know about it before falling back to
threads.

This condition can arise in certain OpenBSD / FreeBSD Python versions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193465 91177308-0d34-0410-b5e6-96231b3b80d8

utils/lit/lit/run.py

@@ -218,6 +218,7 @@ class Run(object):
                 consumer = MultiprocessResultsConsumer(self, display, jobs)
             except ImportError:
                 # Workaround for BSD: http://bugs.python.org/issue3770
+                self.lit_config.note('failed to initialize multiprocessing')
                 consumer = None
         if not consumer:
             task_impl = threading.Thread