When migrating projects to Swift, one does not simply convert codebase at once. Sometimes it just make more sense to write new code in Swift and refactor older ObjC constructs afterwards.
There is a nice documentation provided by Apple, but their interpretation is missing several steps to make all Swifty thing really happen in ObjC project.
Now open your XCode and go to Project / Build Settings. Now do this:
Defines Module : YES
Always Embed Swift Standard Libraries : YES
Install Objective-C Compatibility Header : YES
Clean, Build and add “ModuleName-Swift.h” header to .m files.