go/types, types2: better error message for invalid array length

If an invalid array length is just an identifier, mention
"array length" so that it's clear this is an invalid array
declaration and not a (invalid) generic type declaration.

Fixes #51145.

Change-Id: I8878cbb6c7b1277fc0a9a014712ec8d55499c5c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/385255


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