compiler: connect the concrete type and the placeholder for circular types

compiler: connect the concrete type and the placeholder for circular types

    Previously, when creating the backend representation of a
    circular type, we resolve the placeholder to a
    circular_pointer_type. The backend doesn't know what the
    concrete type is.

    This CL changes it to resolve the placeholder to the concrete
    type instead, so the backend may have better knowledge of the
    concrete type.

    Reviewed-on: https://go-review.googlesource.com/123738

From-SVN: r262830