cmd/compile: erroneously rejects copy(f()) where f() is multi-valued

[mdempsky]

At tip, cmd/compile rejects this package with "missing arguments to copy":

package p
func f() ([]byte, string) { return nil, "" }
var _ = copy(f())

The same package is accepted by go/types and gccgo, and seemingly valid according to the spec.

[mdempsky]

Notably cmd/compile does handle complex(func() (float64, float64) { return 0, 0 }()) correctly.

[ALTree]

This is #15992

[mdempsky]

Closing as dup.