This means more duplication, but it's also more future-proof: the set of
features used is based on the compiler that generates the header, not the
compiler that eventually consumes the header. To allow for forward-
compatibility, all the macros used by the header can be overridden by the
consuming environment, and the header <swift/objc-prologue.h> will be
included if it exists.
Swift SVN r14893
