compiler: use the underlying type to build placeholder type for alias

compiler: use the underlying type to build placeholder type for alias

    When asking for a placeholder type of an alias type, build a
    placeholder for the underlying type, instead of treating the
    alias as a named type and calling get_backend. The latter may
    fail as we may not be ready to build a complete backend type. We
    have already used a unified backend type for alias type and its
    underlying type. Do the same for placeholders as well.

    Reviewed-on: https://go-review.googlesource.com/138635

From-SVN: r264773