cmd/vet: diagnose non-space-separated struct tag like `json:"x",xml:"y"`
This is not strictly illegal but it probably should be (too late)
and doesn't mean what it looks like it means:
the second key-value pair has the key ",xml".
Fixes #14466.
Change-Id: I174bccc23fd28affeb87f57f77c6591634ade641
Reviewed-on: https://go-review.googlesource.com/32031
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Rob Pike <r@golang.org> Reviewed-by: Quentin Smith <quentin@golang.org>