Fix initialization of inner class instances in enum entries

In an inner class of the enum entry class, enum entry reference should
be generated as an outer 'this', not as a enum entry access, because
enum entry itself may be not initialized yet.

compiler/testData/codegen/box/enum/innerClassInEnumEntryClass.kt

@@ -0,0 +1,20 @@
+// LANGUAGE_VERSION: 1.2
+
+enum class A {
+    X {
+        val x = "OK"
+
+        inner class Inner {
+            val y = x
+        }
+
+        val z = Inner()
+
+        override val test: String
+            get() = z.y
+    };
+
+    abstract val test: String
+}
+
+fun box() = A.X.test