Fix enum entry reference from enum entry initialization context

Enum entries are "special" kind of singletons that should be
referenced as a captured 'this' instance inside during entry
initialization, because corresponding static fields in enum class
are not initialized yet.

 #KT-7257 Fixed

compiler/testData/codegen/box/enum/kt7257_anonObjectInit.kt

@@ -0,0 +1,17 @@
+enum class X {
+    B {
+        val value2 = "K"
+
+        val anonObject = object {
+            val value3 = "O" + value2
+
+            override fun toString(): String = value3
+        }
+
+        override val value = anonObject.toString()
+    };
+
+    abstract val value: String
+}
+
+fun box() = X.B.value