cmd/fix: print warning to stderr, show provenance
cmd/fix: print warning to stderr, show provenance

This cgo warning shows up when running "go test" in cmd/fix:

	warning: no cgo types: exit status 1

(It may show up at other times too.)

Warnings belong on standard error, and while we're here,
make it say where it came from.

Change-Id: I3eaba592a219cc6abf2d584762374f9d6ec5135e
Reviewed-on: https://go-review.googlesource.com/c/go/+/240597
Trust: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>

CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,golang/go,cloudflare/go,golang/go,golang/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,cloudflare/go