CL 123362 introduced a bug in creating alias type's backend
    representation. A type's btype_ should not be set before
    named types are converted if it is a placeholder. For alias
    type, it is set too early. This may result in unresolved
    placeholders. This CL fixes it.
    
    Reviewed-on: https://go-review.googlesource.com/123975

From-SVN: r262833