[dev.typeparams] cmd/compile: always generate (*T).M wrappers for instantiated methods

Always generate (*T).M wrappers for instantiated methods, even when the
instantiated method is being generated for another package (its source
package)

Added new function t.IsInstantiated() to check for fully-instantiated
types (generic type instantiated with concrete types, hence concrete
themselves). This function helps hide the representation of instantiated
types outside of the types package.

Added new export/import test setsimp.go that needs this change.

Change-Id: Ifb700db8c9494e1684c93735edb20f4709be5f7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/322193


Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>