We have a pattern where runtimeclasses are defined in MIDL with the expectation that they'll be implemented by multiple other components. In this model the "exclusiveto" interfaces aren't so exclusive since they need to be implemented by other callers. When generating a consuming projection from such a component, we'll pass this new flag so that the exclusiveto interfaces are exposed the same as any other WinRT interface so that consumers can implement those interfaces too.

In this change I added the public_exclusiveto flag and used it in the couple of places that affect the generation of the public interface bits (i.e. helper class visibility and vtable).