cmd/dist: add //line to copied bootstrap files

This way the error messages will show the original file name
in addition to the bootstrap file name, so that you have some
chance of making the correction in the original instead of the copy
(which will be blown away).

Before:
/Users/rsc/g/go/pkg/bootstrap/src/bootstrap/5g/gsubr.go:863: undefined: a

After:
/Users/rsc/g/go/src/cmd/5g/gsubr.go:860[/Users/rsc/g/go/pkg/bootstrap/src/bootstrap/5g/gsubr.go:863]: undefined: a

Change-Id: I8d6006abd9499edb16d9f27fe8b7dc6cae143fca
Reviewed-on: https://go-review.googlesource.com/7741Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/buildtool.go

@@ -139,7 +139,7 @@ func bootstrapFixImports(text, srcFile string) string {
 		}
 	}
 
-	lines[0] = "// Do not edit. Bootstrap copy of " + srcFile + "\n\n" + lines[0]
+	lines[0] = "// Do not edit. Bootstrap copy of " + srcFile + "\n\n//line " + srcFile + ":1\n" + lines[0]
 
 	return strings.Join(lines, "")
 }