cmd/vendor: vendor x/tools once more before the beta

[findleyr]

x/tools contains fixes (namely CL 364678) for vet analyzers as a result of late changes to go/types. It must be vendored once more before the beta.

I can do it, though for good measure I'd like to wait on CL 366614 first.

[gopherbot]

Change https://golang.org/cl/368774 mentions this issue: all: update vendored golang.org/x/tools