[IR] Fix synthetic declarations generator to make it produce correct type

- Fix KT-40126
 - Add test

js/js.translator/testData/box/delegation/delegationToExternaInterface.kt

@@ -0,0 +1,15 @@
+// EXPECTED_REACHABLE_NODES: 1236
+// KJS_WITH_FULL_RUNTIME
+// KT-40126
+@file:Suppress("EXTERNAL_DELEGATION")
+
+@Suppress("NESTED_CLASS_IN_EXTERNAL_INTERFACE")
+external interface MySymbol {
+    companion object : MySymbolConstructor by definedExternally
+}
+external interface MySymbolConstructor {
+    @nativeInvoke
+    operator fun invoke(description: String = definedExternally): Any
+}
+
+fun box() = "OK"