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>