encoding/gob: use reflect.TypeFor for known types

encoding/gob: use reflect.TypeFor for known types

This avoids several mildly confusing Elem calls.

For #60088

Change-Id: If7b83d2ab10537c7e886a035b43cb272130c1669
Reviewed-on: https://go-review.googlesource.com/c/go/+/514455
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>