Support extensions in functional supertypes

Under a flag for now.

compiler/testData/codegen/box/extensionFunctions/extensionFunctionAsSupertype.kt

@@ -0,0 +1,15 @@
+// !LANGUAGE: +FunctionalTypeWithExtensionAsSupertype
+// IGNORE_BACKEND: JS, JS_IR, WASM
+
+interface I: (String) -> String
+
+class C: String.() -> String, I {
+    override fun invoke(p1: String): String = p1
+}
+
+fun box(): String {
+    val c = C()
+    if (c("OK") != "OK") return c("OK")
+    val ext: String.() -> String = c
+    return "OK".ext()
+}