scan-build-py: fix multiprocessing error
Recent versions of python3's multiprocessing module will blow up with
a Runtime error from this code, saying:
An attempt has been made to start a new process before the current process has finished its bootstrapping phase
This is becuae the wrappers in bin/ are not using the __name__ == "__main__" idiom correctly.
Reviewed By: ldionne
Differential Revision: https://reviews.llvm.org/D87051