cmd/compile: fix parameterized interfaces

type I[T any] interface{}

This is an interface, but it has a type parameter.
We need to distinguish that from an interface that is not parameterized.

That means when doing type substitution on an interface with
parameters, we need to make a new one.

Same for non-empty interfaces. Even if the type parameter is not
used in any method, we sill need to make a new type.

Similar case to tstruct, above.

Change-Id: I23ad9f21d2c4ef675bf3f7d84899d9e4919d05e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/344578


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>