go/types, types2: rename allowVersionf to verifyVersionf

go/types, types2: rename allowVersionf to verifyVersionf

Follow-up on comment in CL 491715.

Change-Id: Ie6a71859e791434b7ab53c5524f35718a3567ecb
Reviewed-on: https://go-review.googlesource.com/c/go/+/492236
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>