Support enums in ultra-light classes

There's no need to add "values"/"valueOf" methods for them
(see com.intellij.psi.impl.compiled.StubBuildingVisitor#visitMethod that ignores them too)

We already have tests that check enum entries/synthetic methods
are properly resolved in Java:
idea/testData/kotlinAndJavaChecker/javaAgainstKotlin/*Enum*

core/descriptors/src/org/jetbrains/kotlin/builtins/KotlinBuiltIns.java

@@ -1105,6 +1105,10 @@ public abstract class KotlinBuiltIns {
         return classFqNameEquals(descriptor, FQ_NAMES._enum);
     }
 
+    public static boolean isEnum(@NotNull KotlinType type) {
+        return isConstructedFromGivenClassAndNotNullable(type, FQ_NAMES._enum);
+    }
+
     public static boolean isComparable(@NotNull ClassDescriptor descriptor) {
         return classFqNameEquals(descriptor, FQ_NAMES.comparable.toUnsafe());
     }