go/types, types2: type-check built-ins even if there's a version error

go/types, types2: type-check built-ins even if there's a version error

There is no harm in continuing type-checking a built-in even if there
is a version error.

Change-Id: I161abd904a26075694c26639e247a17126947fcd
Reviewed-on: https://go-review.googlesource.com/c/go/+/496415
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>