sweet/common/diagnostics: gofmt format

Change-Id: Ia7734b408fa24d3abee6f25d47dfa85715084ed9
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/520737
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>