I'm trying to restore Code Sense, which has stopped working for my project, and it looks like the accepted way to do that is to delete the derived data.
The problem is that the project has to be open or it doesn't appear in the Organizer, and the delete button for the derived data is faded out and unable to be clicked.
My project is a dropbox folder.
You can clear Derived Data manually without using Organizer. Go to ~/Library/Developer/Xcode/DerivedData/ and delete every thing there, or at least the folders that start with the name of your project.