Use random names instead of pass name for ML files

Many LLVM passes contain characters that may result in the request
for invalid file names. We instead now use a random file prefix for the
machine learner filestem.

	* ml/ml.cpp (generate_filename): Added to create filename.
	(mageec_ml::make_decision): Use generated filename for temporaries
	instead of passnames.
	(mageec_ml::process_results): Likewise.