Regression test for 'references' bug golang/go#59851.

-- go.mod --
module example.com
go 1.12

-- a/a.go --
package a

type Iface interface {
     Method()
}

type implOne struct{}

func (implOne) Method() {} //@loc(def1, "Method"), refs(def1, def1, ref1, iref, ireftest)

var _ = implOne.Method //@loc(ref1, "Method")
var _ = Iface(nil).Method //@loc(iref, "Method")

-- a/a_test.go --
package a

type implTwo struct{}

func (implTwo) Method() {} //@loc(def2, "Method"), refs(def2, def2, iref, ref2, ireftest)

var _ = implTwo.Method //@loc(ref2, "Method")
var _ = Iface(nil).Method //@loc(ireftest, "Method")