cmd/vet: should not warn +build in raw string literal

[cherrymui]

After CL 60531 (https://go-review.googlesource.com/c/go/+/60531), the misc builder fails with

##### go vet std cmd
go run main.go -p freebsd/arm
go/types/hilbert_test.go:55: +build comment must appear before package clause and be followed by a blank line
exit status 1

(https://build.golang.org/log/890387a04a58201c9d1cf7d819dbe939e3e5d4b0)

The relevant line is https://go-review.googlesource.com/c/go/+/60531/2/src/go/types/hilbert_test.go#55
which is a +build in raw string literal.

[cznic]

I think this is intentional (ie. WAI), because some go tools are known to not parse the source when looking for [certain?] magic comments.

The solution is to, for example

s/+build/+`+`build/

or to use any other way of hiding the directive in it's plain form and starting at BOL.

[cherrymui]

If this is the case we probably need to change go/types/hilbert_test.go to work around it. Kind of unfortunate.

[gopherbot]

Change https://golang.org/cl/60776 mentions this issue: go/types: escape +build sequence to silence vet warning

[mdempsky]

We've worked around this issue in hilbert_test.go for now to appease the vet builder. If we decide to fix in cmd/vet instead, we should undo the workaround.

[robpike]

Working as intended.