types2: use strings.Builder in infer.go

types2: use strings.Builder in infer.go

Permissible now because go1.17 is used for bootstrapping.

Change-Id: Ic97b0706190c3eb907da69003bc94448c90e5af7
Reviewed-on: https://go-review.googlesource.com/c/go/+/427556
Run-TryBot: Robert Griesemer <gri@google.com>
Run-TryBot: hopehook <hopehook@golangcn.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>