Reset "_last" for all but first enum elements.

There is an implicit assumption (see TypePass::enumvalueDeclaration()) that
only the first enum element has a non-null "_last" attribute, but this was
broken by the latest enum-related grammar changes as the second enum element
also had "_last" set, coming from the new "enumlist_tail" production. This
resulted in wrong values being used for the second (only) element.

Fix this by explicitly resetting "_last" of enumlist_tail to NULL when
building the semantic value associated with it.

Source/CParse/parser.y

@@ -5944,6 +5944,7 @@ enumlist	: enumlist_item optional_ignored_define_after_comma {
 		| enumlist_item enumlist_tail optional_ignored_define_after_comma {
 		  set_nextSibling($1, $2);
 		  Setattr($1,"_last",Getattr($2,"_last"));
+		  Setattr($2,"_last",NULL);
 		  $$ = $1;
 		}
 		| optional_ignored_define {