vet's buildtag check looks for malformed build tag comments. Since these
can appear in Go files as well as non-Go files (such as assembly files),
it must read the file line by line instead of using go/token or go/ast
directly.

However, this method runs into false positives if there are any lines in
the code that look like comments, but are not. For example:

	$ cat f.go
	package main
	const foo = `
	//+build ignore
	`
	$ go vet f.go
	./f.go:3: +build comment must appear before package clause and be followed by a blank line

This bug has been popping up more frequently since vet started being run
with go test, so it is important to make the check as precise as
possible.

To avoid the false positive, when checking a Go file, cross-check that a
line that looks like a comment actually corresponds to a comment in the
go/ast syntax tree. Since vet already obtains the syntax trees for all
the Go files, it checks, this change means very little extra work for
the check.

While at it, add a badf helper function to simplify the code that
reports warnings in the buildtag check.

Fixes #13533.

Change-Id: I484a16da01363b409ec418c313634171bf85250b
Reviewed-on: https://go-review.googlesource.com/111415
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>