In Swift 3, an Objective-C type like SomeClass <SomeProtocol> is
imported as SomeClass. The protocol qualification is erased unless
the class bound is 'id' or 'Class'.
Importing such types as class-constrained existentials is a source
breaking change, so the new behavior is only enabled in Swift 4
mode.
Furthermore as a transitional step the staging flag
-enable-experimental-subclass-existentials has to be passed in
also. The flag will soon be removed.
