IR JS: support findAssociatedObject feature (KT-37418 fixed)

js/js.translator/testData/box/reflection/findAssociatedObject_oldBE.kt

@@ -0,0 +1,22 @@
+// EXPECTED_REACHABLE_NODES: 1321
+// IGNORE_BACKEND: JS_IR
+
+import kotlin.reflect.*
+
+@OptIn(ExperimentalAssociatedObjects::class)
+@AssociatedObjectKey
+@Retention(AnnotationRetention.BINARY)
+annotation class Associated1(val kClass: KClass<*>)
+
+@Associated1(Bar::class)
+class Foo
+
+object Bar
+
+@OptIn(ExperimentalAssociatedObjects::class)
+fun box(): String {
+    // This API is not implented in the old backend.
+    if (Foo::class.findAssociatedObject<Associated1>() != null) return "fail 1"
+
+    return "OK"
+}