compiler: build a single backend type for type alias

compiler: build a single backend type for type alias

    A type alias and its underlying type are identical. Build a
    single backend type for them. Previously we build two backend
    types, which sometimes confuse the backend's type system.

    Also don't include type aliases into the list of named type
    declarations, since they are not named types.

    Reviewed-on: https://go-review.googlesource.com/123362

From-SVN: r262572