This has been confirmed as a false positive and had already sorted itself out Saturday afternoon (along with a Dr Web false positive as well).  As of the time of writing, Windows Defender should now have no problems with the free version of CCleaner 5.70. We get this from time to time with new releases of CCleaner - an AV company's threat detection robots will observe that something they haven't seen before is suddenly getting millions of downloads in a day and they block it "just in case"