Insert a #warning if generating a setPrimitiveType: accessor

This is a suggested alternative to the fix in commit cd9809d. That fix results in the `primitiveType` and `setPrimitiveType:` accessors being silently suppressed. This fix still generates them (and hence doesn't break a build), but adds a #warning line (which Xcode will flag up) indicating that there's a problem if the app is intended for App Store submission.