[release-branch.go1.13] cmd: update golang.org/x/tools to v0.0.0-20200615191743-991d59a616de

This teaches vet to recognize the new formats %#b and %O on integers,
%x on float/complex. They have been supported since the 1.13 release,
but vet would warn about it.

Also update cmd/vet testdata accordingly, as was done in CL 202083.

Fixes #39287.
For #29986.

Change-Id: Ia7817ee60ae6beac32cc402c0c68afa917e4ef0f
Reviewed-on: https://go-review.googlesource.com/c/go/+/238018


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>