cmd/compile/internal/types2: mark implicit interfaces as such

cmd/compile/internal/types2: mark implicit interfaces as such

Provide an accessor for clients, and don't print the interface
around implicitly wrapped embedded types.

For #48424.

Change-Id: Ib2c76315508fc749ea4337d52e13d17de80e04da
Reviewed-on: https://go-review.googlesource.com/c/go/+/353396
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>