Make enum fail with a NYI error

The enum handling tries to rebless the value to the enum.  Clearly,
one cannot rebless a type object into something else.  Now, whether
it should be possible to have a type object as the value of an enum
I'm letting open for now by making it a NYI error.

Skarsnik++ for spotting

src/Perl6/Actions.nqp

@@ -4302,6 +4302,11 @@ class Perl6::Actions is HLL::Actions does STDActions {
             unless nqp::istype($cur_key, $*W.find_symbol(['Str'])) {
                 $cur_key := $cur_key.Str;
             }
+            unless nqp::defined($cur_value) {
+                $*W.throw($/, 'X::Comp::NYI',
+                    feature => "Using a type object as a value for an enum",
+                );
+            }
 
             # Create and install value.
             my $val_obj := $*W.create_enum_value($type_obj, $cur_key, $cur_value);