Xcode update would not work, and now won't leave the App Store update queue
I was able to fix this problem by correcting the permissions for Xcode. Run this from your Terminal:

sudo pkgutil --repair com.apple.pkg.Xcode


It actually installed as I was doing this. Many of the permissions reported as "should be drwxrwxr-x , they are drwxr-xr-x".