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>

src/cmd/vet/structtag.go

@@ -34,7 +34,8 @@ func checkCanonicalFieldTag(f *File, node ast.Node) {
 	}
 
 	if err := validateStructTag(tag); err != nil {
-		f.Badf(field.Pos(), "struct field tag %s not compatible with reflect.StructTag.Get: %s", field.Tag.Value, err)
+		raw, _ := strconv.Unquote(field.Tag.Value) // field.Tag.Value is known to be a quoted string
+		f.Badf(field.Pos(), "struct field tag %q not compatible with reflect.StructTag.Get: %s", raw, err)
 	}
 
 	// Check for use of json or xml tags with unexported fields.