[WasmJs] Add support for external class reflection

Fix #KT-64890

compiler/testData/codegen/boxWasmJsInterop/associatedExternalObject.kt

@@ -0,0 +1,25 @@
+// TARGET_BACKEND: WASM
+
+// FILE: findAssociatedExternalObject.js
+
+const JS_OBJECT = {}
+
+// FILE: findAssociatedExternalObject.kt
+
+import kotlin.reflect.*
+
+@OptIn(ExperimentalAssociatedObjects::class)
+@AssociatedObjectKey
+@Retention(AnnotationRetention.BINARY)
+annotation class Associated(val kClass: KClass<*>)
+
+external object JS_OBJECT
+
+@Associated(JS_OBJECT::class)
+class ObjectKey
+
+@OptIn(ExperimentalAssociatedObjects::class)
+fun box(): String {
+    if (ObjectKey::class.findAssociatedObject<Associated>() !== JS_OBJECT) return "FAIL2"
+    return "OK"
+}