go/types: restore original assignment error messages

This is the missing portion of the port of CL 351669
from types2 to go/types, now that we have a local flag
to control for compiler error messages.

Mostly a clean port but for adjustments to error reporting
which requires error codes in go/types.

Prerequisite for port of CL 364874.

Change-Id: I5fc8c83003e4396351f42e9adb08f4ebc8a05653
Reviewed-on: https://go-review.googlesource.com/c/go/+/367195


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>