Ok, I have to confirm: I went back to the Mac after about 5 hours, tried to run XCode, and this time it started. If you run into this problem, you may want to give the XCode 1 to 5 hours to update.

Identical issue here, after there was a Command Line Tools for XCode update. Rebooted 4 times over the course of 30 minutes. I suppose I will now wait 3 to 4 hours. Very annoying.