Added test on function overriding return type with Unit

backend.native/tests/codegen/bridges/test18.kt

@@ -0,0 +1,13 @@
+// overriden function returns Unit
+open class A {
+    open fun foo(): Any = 42
+}
+
+open class B: A() {
+    override fun foo(): Unit { }
+}
+
+fun main(args: Array<String>) {
+    val a: A = B()
+    println(a.foo())
+}