go/printer: error out of Fprint when it would write a '//line' directive with a multi...

go/printer: error out of Fprint when it would write a '//line' directive with a multiline file path

Line directives do not provide a way to escape newline characters, so
source file paths containing newlines must not be written in them.

Updates #60167.

Change-Id: I30f8b381cc7d1df6914c27591544edf424a4b634
Reviewed-on: https://go-review.googlesource.com/c/go/+/501578
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>