debug: use strings.Builder
debug: use strings.Builder

Change-Id: I59db11212758b478248f34c031501a167335348c
Reviewed-on: https://go-review.googlesource.com/c/go/+/428257
Auto-Submit: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go