I have started to see same errors after Xcode 6.1 and iOS 8.1 update. I have found that if you delete ModuleCache file at /Users/username/Library/Developer/Xcode/DerivedData/ModuleCache path, it fixes the error temporarily. And you don’t have to close Xcode or the project while doing that. After deleting the file, just wait for Xcode a little to index the project files again. After that, the problem mostly resolves for some time.
As it doesn’t solve the problem permanently, you should avoid writing codes causing this according to other answers till Apple solves this.