Some enum-related fixes

(NB: testEnumClass3 doesn't work yet, effective modality required for enum class)

compiler/testData/ir/box/enumClass3.kt

@@ -0,0 +1,17 @@
+fun <T> assertEquals(actual: T, expected: T) {
+    if (actual != expected) {
+        throw java.lang.AssertionError("Assertion failed: $actual != $expected")
+    }
+}
+
+enum class TestEnumClass {
+    ZERO {
+        override fun describe() = "nothing"
+    };
+    abstract fun describe(): String
+}
+
+fun box(): String {
+    assertEquals(TestEnumClass.ZERO.describe(), "nothing")
+    return "OK"
+}