Don't fail when using Unit class object as a value

#KT-4243 Fixed

compiler/testData/codegen/box/unit/unitClassObject.kt

@@ -0,0 +1,13 @@
+fun box(): String {
+    Unit
+
+    val a = Unit
+    val b = Unit
+    if (a != b) return "Fail a != b"
+
+    if (Unit != Unit) return "Fail Unit != Unit"
+
+    if (a.VALUE != Unit.VALUE) return "Fail a.VALUE != Unit.VALUE"
+
+    return "OK"
+}