encoding/json: make use of reflect.Type.{OverflowInt, OverflowUint}

encoding/json: make use of reflect.Type.{OverflowInt, OverflowUint}

CL 567296 added {OverflowComplex, OverflowFloat, OverflowInt, OverflowUint}
to reflect.Type, this CL uses these methods to simplify code.

For #60427

Change-Id: I229aef9e4095a2f025afd782081f6c9e6d7710f3
GitHub-Last-Rev: c824e5a1b5547e2cc23142fbcf0d6dd59f0e8506
GitHub-Pull-Request: golang/go#66000
Reviewed-on: https://go-review.googlesource.com/c/go/+/567775
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>