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>
5 files changed
tree: 476975500ec19e72f415959830f35b30dbf44c91
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. AUTHORS
  11. CONTRIBUTING.md
  12. CONTRIBUTORS
  13. favicon.ico
  14. LICENSE
  15. PATENTS
  16. README.md
  17. robots.txt
  18. SECURITY.md
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