compiler: generate type descriptor for pointer to alias defined in another package

compiler: generate type descriptor for pointer to alias defined in another package

    When a type descriptor is needed (for e.g. interface conversion),
    if the type is a pointer to a named type defined in another
    package, we don't generate the definition of the type descriptor
    because it is generated in the package where the type is defined.
    However, if the named type is an alias to an unnamed type, its
    descriptor is not generated in the other package, and we need to
    generate it.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/210787

From-SVN: r279207