Fixes#5826
The `prevent_destroy` lifecycle configuration was not being checked when
the count was decreased for a resource with a count. It was only
checking when attributes changed on pre-existing resources.
This fixes that.
When the `prevent_destroy` flag is set on a resource, any plan that
would destroy that resource instead returns an error. This has the
effect of preventing the resource from being unexpectedly destroyed by
Terraform until the flag is removed from the config.
