[dev.typeparams] cmd/compile: add built-in name/type "comparable".

This allows exporting comparable type bounds, and importing back into
types2 for typechecking.

Fixes typeparam/mdempsky/8.go

Change-Id: I3ee12433df2ed68ac6ef4cad24be9fcdfaaca4e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/333129


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>