Xcode 6.3 is out now and requires 10.10. I also waited as long as I could, so now I have a machine that's been installing Yosemite for the last 18 hours...
It's a known issue related to having many nonstandard files in /usr/local or something like that. Eg. If you are using brew. Google for it, or just wait patiently, it should finish at some point.