Place inner enum into its parent's class object

#KT-2771 In Progress

ClassifierCollector now collects all enum declarations inside non-static
classes. After all declarations are done, it processes these enums: creates
a class object if it's not present, reports errors on each enum if a class
object is not allowed, and puts every enum into this class object otherwise,
recording this fact to trace.

compiler/testData/codegen/enum/innerWithExistingClassObject.kt

@@ -0,0 +1,8 @@
+class A {
+    class object {}
+    enum class E {
+        OK
+    }
+}
+
+fun box() = A.E.OK.toString()