go/internal/gcimporter: simplify unified IR importer
    
    CL 424854 changed the unified IR writer's handling of type
    declarations to write the underlying type rather than the RHS type
    expression's type. This in turn allows us to simplify the go/types
    importer, because now there's no need to delay caling SetUnderlying.
    
    Fixes #57015.
    
    Change-Id: I80caa61f6cad5b7f9d829939db733a66cfca621c
    Reviewed-on: https://go-review.googlesource.com/c/go/+/424876
    Reviewed-by: Robert Griesemer <gri@google.com>
    Run-TryBot: Matthew Dempsky <mdempsky@google.com>
    TryBot-Result: Gopher Robot <gobot@golang.org>
    Reviewed-by: Robert Findley <rfindley@google.com>