cmd/compile/internal/types2: implement copy for generic argument types

cmd/compile/internal/types2: implement copy for generic argument types

For now, the underlying types of the the argument types' constraints
must be a single type that is a slice (the source operand may also
be a string).

Change-Id: I9e705e3349c9242f126b9c3e0af65e9ffb25fe6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/354432
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>