cmd/vet: include the errors.As check from upstream x/tools

This change revendors golang.org/x/tools to include the check and
modifies cmd/vet to add it to the command.

CL 179977 will enable the check by default for 'go test'.

Commands run (starting in GOROOT/src):
	cd cmd
	emacs vet/main.go
	go get -u=patch golang.org/x/tools/go/analysis/passes/errorsas@latest
	go mod tidy
	go mod vendor
	cd ..
	./make.bash
	go test all

Updates #31213

Change-Id: Ic2ba9bd2d31c4c5fd9e7c42ca14e8dc38520c93b
Reviewed-on: https://go-review.googlesource.com/c/go/+/181717


Reviewed-by: Jonathan Amsterdam <jba@google.com>