Treat anonymous objects as classes (ClassKind.CLASS)

They're not objects per se, i.e. they're not singletons

compiler/testData/writeFlags/class/accessFlags/objectLiteral.kt

@@ -0,0 +1,7 @@
+class Foo {
+    val x = object { }
+}
+
+// TESTED_OBJECT_KIND: class
+// TESTED_OBJECTS: Foo$x$1
+// FLAGS: ACC_PUBLIC, ACC_FINAL, ACC_SUPER