LeakingThisInspection: fix false positives for enum class

#KT-25905 Fixed

idea/testData/inspectionsLocal/leakingThis/enumEntryHasOverriddenMember3.kt

@@ -0,0 +1,14 @@
+// PROBLEM: Calling non-final function toString in constructor
+// FIX: none
+enum class Fo(val b: Int) {
+    ONE(5) {
+        override val x = b
+        override fun toString(): String {
+            return x.toString()
+        }
+    };
+
+    abstract val x: Int
+
+    val dos = <caret>toString()
+}