go/types: provide a better error message for [...] array types

go/types: provide a better error message for [...] array types

This matches types2 behavior.

For #54511.

Change-Id: Iea906e9fec7e334b7aa7f481de87373fa93d1c7c
Reviewed-on: https://go-review.googlesource.com/c/go/+/425715
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>