I ran into this bugger too.
I was running an older version of Xcode (not compatible with ios 9.2) so I needed to update.
I spent hours on this and was constantly getting spinning wheel of death in the app store. Nothing worked. I tried CLI softwareupdate, updating OSX, everything.
I ultimately had to download AppZapper, then nuked XCode.
I went into the app store to download and it still didn't work. Then I rebooted.
And from here I could finally upgrade to a fresh version of xcode.
WARNING: AppZapper can delete all your data around Xcode as well, so be prepared to start from scratch on your profiles, keys, etc. Also per the other notes here, of course be ready for a 3-5 hour long downloading expedition...