NSObject related compile issue on Linux

[Slava_Pestov]

@Kyle-Ye

Got it. I'll try to read the relevant compiler code and revisit it later.

I went looking for the code and it looks like the relevant logic is encoded in canInheritDesignatedInits() which calls hasUserDefinedDesignatedInit() and areAllStoredPropertiesDefaultInitializable(). Both return early if the class is imported from Objective-C.