go/types, types2: use strings.Builder instead of bytes.Buffer where possible

go/types, types2: use strings.Builder instead of bytes.Buffer where possible

Also, consistently use declaration: var buf strings.Builder.
We don't change exported signatures to match go/types (where we
can't change the exported signatures for backward-compatibility).

Change-Id: I75350886aa231889ae2fd5c4008dd4be9ed6e09f
Reviewed-on: https://go-review.googlesource.com/c/go/+/428094
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>