cmd/vet: add directive analyzer

cmd/vet: add directive analyzer

For #56986, add the new directive analyzer that catches
misplaced //go:debug lines.

Ran 'go mod vendor' after adding the import in vet
to bring in the vendored files.

A followup CL will enable it by default in 'go test'.

Change-Id: I12c46e292b31bdbf5ceb86ba4474545e78a83a47
Reviewed-on: https://go-review.googlesource.com/c/go/+/462201
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>