go/parser: include more comments in a struct or interface

go/parser: include more comments in a struct or interface

While parsing inside a struct or an interface, skipping over empty lines
too to collect the next group of comments. We do not need to skip
over more than 1 empty line since gofmt already removes multiple
empty consecutive lines.

Fixes #10858

Change-Id: I0c97b65b5fc44e225e5dc7871ace24f43419ce08
Reviewed-on: https://go-review.googlesource.com/c/go/+/161177
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>