x/mobile: circular dependencies between interfaces won't build on ios targets

[benbenbenbenbenben]

We had a particular case where Objective-C would have needed a forward declaration when building the following Go snippets as Objective-C:

type BuilderOptionContext interface {
	// public API
        // (omitted for brevity)
	// private API
	getOptions() *[]BuilderOption
}

type BuilderOption interface {
	Option() BuilderOptionType
	Apply(builder BuilderOptionContext) error
}

It was easier enough to work around but there was no indication from gomobile what the problem was. The native build failed reporting a type was expected at line/column but no more detail than that. It was only after I commented out the cleanup in cmd/gomobile/bind/go that I could see the generated source and work out what was happening... perhaps we're missing a flag to prevent cleanup on failure?

Granted this was a code smell and we don't put any private API on that public interface now, but it was interesting nevertheless.

[cagedmantis]

cc @hajimehoshi