The LR6 catalog file, .lrcat, no longer shows Lightroom's icon, does not appear under HK_CLASSES_ROOT in the registry, nor on the Windows file association list. Having said that, LR6/CC loads the catalog normally and can be started by double-clicking the .lrcat file. So, the question is probably purely academic as functionality does not seem to be impaired.
(In my case, LR CC has been installed on two machines, replacing LR5.7: desktop running Win7 and laptop running Win8.1. LR6's (upgraded) .lrcat does not show as the default LR file on either machine. Lightroom 5's catalog did.)