cmd/vet: false positive on +build comment if within a string

[andybons]

go version go1.10 linux/amd64

When running go vet on the following code, it complains that the +build comment is in the wrong place:

package main

const prog = `
// +build foo
package main

func main() {}
`

[robpike]

It's tricky and maybe hard to fix but I'll look into it.
The issue is that vet is looking for things in the wrong places, things the parser will toss, so it can't use the parser, yet without parsing the code it's easy to mislead the scanner.

[andybons]

Thanks, Rob.

[0xmohit]

This was reported earlier here: #13533

[robpike]

Duplicate of #13533.

That one was closed as too hard to fix for the payoff, which is fair. Closing.