[dev.typeparams] cmd/compile: fix some issues with cons.go

Add a test to make sure there's no invalid OCONVIFACEs when stenciling is done.

Use concrete types for the type of DOTTYPE and DOTTYPE2.

MarkTypeUsedInInterface - should we allow types with shape types
underneath? I think the itab CL will help with this (at least, for
a remaining cons.go issue).

Change-Id: I2c96d74e8daaca26cadc84ea94abb9a27c0bb240
Reviewed-on: https://go-review.googlesource.com/c/go/+/337069


Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>