Replace any nested Writer that is a MultiWriter with its associated
Writers.

Fixes #22431

Change-Id: Ida7c4c83926363c1780689e216cf0c5241a5b8eb
Reviewed-on: https://go-review.googlesource.com/73470
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>