cmd/vet: update buildtag check for //go:build lines

Brings in golang.org/x/tools@2363391a
and adjusts, adds cmd/vet tests accordingly.

Part of //go:build change (#41184).
See https://golang.org/design/draft-gobuild

This brings in the new //go:build checks in cmd/vet.

Change-Id: I8a9735cc014171691012b307ec30e94c81aadfe1
Reviewed-on: https://go-review.googlesource.com/c/go/+/240609


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>