cmd/compile: remove totype0 type-constructor helpers
cmd/compile: remove totype0 type-constructor helpers

These were originally introduced for the binary export format, which
required forward references to arbitrary types and later filling them
in. They're no longer needed since we switched to the indexed export
format, which only requires forward references to declared types.

Passes toolstash-check.

Change-Id: I696dc9029ec7652d01ff49fb98e658a9ed510979
Reviewed-on: https://go-review.googlesource.com/c/go/+/228579
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Reviewed-by: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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