go/types, types2: report "undefined: x" instead of "undeclared name: x"

go/types, types2: report "undefined: x" instead of "undeclared name: x"

This matches the compiler's long-standing behavior.

For #55326.

Change-Id: I90696a11f0b7d1f4be95a4b9a6f01844df2a2347
Reviewed-on: https://go-review.googlesource.com/c/go/+/432555
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>