Some enum-related fixes

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

compiler/testData/ir/box/enumClass2.kt

@@ -0,0 +1,21 @@
+fun <T> assertEquals(actual: T, expected: T) {
+    if (actual != expected) {
+        throw java.lang.AssertionError("Assertion failed: $actual != $expected")
+    }
+}
+
+enum class TestEnumClass(val x: Int) {
+    ZERO
+//    {
+//        init {
+//        }
+//    }
+    ;
+    constructor(): this(0)
+}
+
+fun box(): String {
+    assertEquals(TestEnumClass.ZERO.x, 0)
+
+    return "OK"
+}