Use domain_search_flags in lookup_global_symbol_language

This changes quick_symbol_functions::lookup_global_symbol_language to
accept domain_search_flags rather than just a domain_enum, and fixes
up the fallout.

To avoid introducing any regressions, any code passing VAR_DOMAIN now
uses SEARCH_VFT.

That is, no visible changes should result from this patch.  However,
it sets the stage to refine some searches later on.

gdb/language.h

@@ -369,7 +369,7 @@ struct language_defn
      used as the definition.  */
   virtual bool iterate_over_symbols
 	(const struct block *block, const lookup_name_info &name,
-	 domain_enum domain,
+	 domain_search_flags domain,
 	 gdb::function_view<symbol_found_callback_ftype> callback) const
   {
     return ::iterate_over_symbols (block, name, domain, callback);