Add a generation option to control use of forward declarations in headers. - #9568

…ders.

Swift importing ObjC drops methods/properties if the type is only a forward
declaration since the type is incomplete. Historically the generator has always
use forward declarations to reduce how much will have rebuild when a proto file
does change; but that puts it at odds with Swift. If ObjC Protos end up spanning
Swift modules, the Swift import behavior could become a problem; so this option
provides a control for the behavior. The current behavior is to continue forward
declarations, but eventually the default will be changed.

Generate the WKTs using imports instead of forward decls.