[JS IR] Properly detect whether nested class is external

- Fix KT-38765

compiler/testData/ir/irJsText/external/kt38765.kt

@@ -0,0 +1,18 @@
+package events
+
+external open class internal {
+    open class EventEmitterP : internal {
+    }
+
+    open class EventEmitterS : internal {
+        constructor(a: Any)
+    }
+
+    object NestedExternalObject : internal {}
+
+    enum class NestedExternalEnum {
+        A, B
+    }
+
+    interface NestedExternalInterface {}
+}