Test of "undeclared" diagnostic in generated code.

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

-- generated.go --
package generated

// Code generated by generator.go. DO NOT EDIT.

func _() {
	var y int //@diag("y", re"y declared (and|but) not used")
}

-- generator.go --
package generated

func _() {
	var x int //@diag("x", re"x declared (and|but) not used")
}