The upshot of this is that internal decls in an app target will be in the
generated header but internal decls in a framework target will not. This
is important since the generated header is part of a framework's public
interface. Users always have the option to add members via category to an
internal framework type they need to use from Objective-C, or to write the
@interface themselves if the entire type is missing. Only internal protocols
are left out by this.
The presence of the bridging header isn't a /perfect/ way to decide this,
but it's close enough. In an app target without a bridging header, it's
unlikely that there will be ObjC sources depending on the generated header.
Swift SVN r19763
