cmd/cgo: error out if the source path used in line directives would contain a newline

cmd/cgo: error out if the source path used in line directives would contain a newline

cmd/cgo uses '//line' directives to map generated source
files back to the original source file and line nmubers.

The line directives have no way to escape newline characters,
so cmd/cgo must not be used if the line directives would contain
such characters.

Updates #60167.

Change-Id: I8581cea74d6c08f82e86ed87127e81252e1bf78c
Reviewed-on: https://go-review.googlesource.com/c/go/+/501576
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>