cmd/compile: fix printing of untyped types in -W output
cmd/compile: fix printing of untyped types in -W output

It's always useful to distinguish "bool" and "string" from "untyped
bool" and "untyped string", so change typefmt to do this
unconditionally.

Also, while here, replace a bare 0 with its named constant FErr.

Fixes #23833.

Change-Id: I3fcb8d7204686937439caaaf8b3973fc236d0387
Reviewed-on: https://go-review.googlesource.com/94021
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/go,Cofyc/go,codestation/go,golang/go,cloudflare/go,nawawi/go,akutz/go,nawawi/go,codestation/go,nawawi/go,josharian/go,nawawi/go,akutz/go,golang/go,josharian/go,akutz/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,josharian/go,Cofyc/go,Cofyc/go,nawawi/go,Cofyc/go,codestation/go,cloudflare/go,Cofyc/go,codestation/go,cloudflare/go,golang/go,josharian/go,codestation/go,akutz/go,CAFxX/go,CAFxX/go,golang/go,akutz/go,josharian/go,Cofyc/go,akutz/go,nawawi/go,codestation/go,cloudflare/go,josharian/go,golang/go,Cofyc/go,codestation/go,golang/go,akutz/go,codestation/go,akutz/go,cloudflare/go,golang/go,josharian/go,CAFxX/go,CAFxX/go,Cofyc/go,CAFxX/go,josharian/go,CAFxX/go,cloudflare/go,nawawi/go