[JS IR] Fix IrGetClass for Throwable successors

- add test

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

@@ -0,0 +1,13 @@
+// EXPECTED_REACHABLE_NODES: 1282
+
+
+fun Throwable.className() = this::class.simpleName!!
+
+class O : Error()
+class K : Error()
+
+fun box(): String {
+    val o = O()
+    val k = K()
+    return o.className() + k.className()
+}