LeakingThisInspection: fix false positives for enum class

#KT-25905 Fixed

idea/testData/inspectionsLocal/leakingThis/enumEntryHasOverriddenMember4.kt

@@ -0,0 +1,11 @@
+// PROBLEM: none
+// FIX: none
+enum class Fo(val b: Int) {
+    ONE(5) {
+        override val x = b
+    };
+
+    abstract val x: Int
+
+    val dos = <caret>toString()
+}