[dev.typeparams] cmd/compile: mark methods of instantiated interface types as used

Fix the cons.go missing method error. Mark all the methods of
instantiated interface types as used. We could try to record all the
exact methods used for generic interface types, but for now, just mark
all the methods as used so that their methods are not dead-code
eliminated.

Change-Id: I35685eda82476244371379b97691a1b8506ef0f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/337349


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