[LC] add isEquivalentTo implementation so LC can be referenced from java

otherwise references from java to ktElements won't be treated as references,
cause java references resolve to light elements and search can/should be called
on ktElements

analysis/symbol-light-classes/testData/equivalentTo/EnumClassEntry.kt

@@ -0,0 +1,8 @@
+// org.jetbrains.kotlin.light.classes.symbol.classes.SymbolLightClassForEnumEntry
+enum class FooBar {
+    EnumClass<caret>Entry {
+        fun foo() {
+
+        }
+    }
+}