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_boundReference2.kt

@@ -0,0 +1,18 @@
+enum class X {
+    B {
+
+        override val value = "OK"
+
+        val bmr = B::value.get()
+
+        override fun foo(): String {
+            return bmr
+        }
+    };
+
+    abstract val value: String
+
+    abstract fun foo(): String
+}
+
+fun box() = X.B.foo()