I have been investigating this issue for some time too. I think one solution (which I need to credit the stackoverflow user justin for) is to set several config values for Xcode. It should be done after quiting the Xcode of course.
In the terminal execute the following commands:
defaults write com.apple.dt.Xcode IDEBuildOperationMaxNumberOfConcurrentCompileTasks 4
defaults write com.apple.dt.XCode IDEIndexDisable 1
this will prevent indexing and reduce the number of compile tasks. This allowed my Xcode to leave me some RAM for actually doing some work.
to delete these settings, eg indexing:
defaults delete com.apple.dt.XCode IDEIndexDisable As mentioned, this was discussed in this question.
In addition, cleaning the Xcode cache allowed to reduce the speed that the RAM was eaten with. (at ~/Library/Developer/Xcode/DerivedData - it is at your home dir, and also it is probably hidden, so easier to access with Terminal).
