cmd/vet: add ifaceassert and stringintconv checks

cmd/vet: add ifaceassert and stringintconv checks

This change re-vendors x/tools to add the ifaceassert and stringintconv
checks to cmd/vet.

Fixes #32479.
Updates #4483.

Change-Id: I6bd30b0a3278592dfab4bd247036404ddaff09e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/221339
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>