go/types, types2: better error message for some invalid integer array lengths

go/types, types2: better error message for some invalid integer array lengths

Don't say "array length must be integer" if it is in fact an integer.

Fixes #59209

Change-Id: If60b93a0418f5837ac334412d3838eec25eeb855
Reviewed-on: https://go-review.googlesource.com/c/go/+/479115
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>