go/types: assign error codes to new errors for Go 1.18

go/types: assign error codes to new errors for Go 1.18

During development, we used placeholder _Todo error codes for new
errors related to generics. Add real error codes in these places.

As a result, 9 new error codes are added for ~50 call sites.

Change-Id: Ib57b4cd9f0a2e160971a3aeea18f9fe26fc0f835
Reviewed-on: https://go-review.googlesource.com/c/go/+/363874
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>