cmd/vet: re-quote struct tags when printing errors

cmd/vet: re-quote struct tags when printing errors

cmd/link/link_test.go contains several multi-line
struct tags. Going through an unquote/quote cycle
converts `a
b
c` to "a\nb\nc\n".

This keeps all vet error messages for the standard
library on a single line.

Updates #11041

Change-Id: Ifba1e87297a5174294d1fbf73463fd3db357464f
Reviewed-on: https://go-review.googlesource.com/27129
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>