go/printer: canonicalize //go:build and // +build lines while formatting

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

Gofmt and any other go/printer-using program will now:

 - move //go:build and //+build lines to the appropriate file location
 - if there's no //go:build line, add one derived from the // +build lines
 - if there is a //go:build line, recompute and replace any // +build lines
   to match what the //go:build line says

For Go 1.17.

Change-Id: Ide5cc3b4a07507ba9ed6f8b0de846e840876f49f
Reviewed-on: https://go-review.googlesource.com/c/go/+/240608


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