Current CommentedNode cannot handle associated comments which satisfy
    node.End() < comment.Pos()

This CL solves it.

Fixes #20635

Change-Id: I58e2e3703999bb38a6ce37112e986c4b1b2eace0
Reviewed-on: https://go-review.googlesource.com/45292
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>