Throw useful error on attribute in where…

…when trying to use such a `where` to constrain an attribute.

Fixes RT#130748: https://rt.perl.org/Ticket/Display.html?id=130748

If we're declaring a `has` variable, null $*HAS_SELF when we're
parsing the `where` clause for it.

src/Perl6/Grammar.nqp

@@ -2752,7 +2752,9 @@ grammar Perl6::Grammar is HLL::Grammar does STD {
         ]?
 
         [ <.ws> <trait>+ ]?
-        [ <.ws> <post_constraint('var')>+ ]?
+        [ <.ws> :my $*HAS_SELF :=
+              $*SCOPE eq 'has' ?? nqp::null !! nqp::getlexdyn('$*HAS_SELF')
+          ; <post_constraint('var')>+ ]?
     }
 
     proto token routine_declarator { <...> }