Add some new tests on functions and extension functions

compiler/testData/codegen/box/functions/invoke/castFunctionToExtension.kt

@@ -0,0 +1,11 @@
+fun box(): String {
+    val f = fun (s: String): String = s
+    val g = f as String.() -> String
+    if ("OK".g() != "OK") return "Fail 1"
+
+    val h = fun String.(): String = this
+    val i = h as (String) -> String
+    if (i("OK") != "OK") return "Fail 2"
+
+    return "OK"
+}