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/innerClassMethodInEnumEntryClass2.kt

@@ -0,0 +1,19 @@
+// LANGUAGE_VERSION: 1.2
+
+enum class A {
+    X {
+        val x = "OK"
+
+        inner class Inner {
+            fun foo() = this@X.x
+        }
+
+        val z = Inner()
+
+        override val test = z.foo()
+    };
+
+    abstract val test: String
+}
+
+fun box() = A.X.test