cmd/vet: ignore build comments in string literals

[josharian]

package p

const buildtag = `
// +build abc
`

$ go tool vet x.go
x.go:4: +build comment must appear before package clause and be followed by a blank line

This is a detectable false positive. This is very unlikely to be an actual attempt at a build comment; it's inside a string literal. This happens when writing code generators. There's a concrete instance in TestBinaryOnlyPackages in cmd/go/go_test.go.

cc @valyala @robpike

[0xmohit]

Also reported earlier.

[ianlancetaylor]

Closing as dup of #13533.