n.Pos.IsKnown() is not needed because it is performed in setlineno.

toolstash-check passed.

Updates #19683.

Change-Id: I34d6a0e6dc9970679d99e8f3424f289ebf1e86ba
Reviewed-on: https://go-review.googlesource.com/114915
Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>