c++: share more trees representing enumerators

This came up in Reflection where an assert fails because we have two
different trees for same enumerators.  The reason is that in
finish_enum_value_list we copy_node when converting the enumerators.
It should be more efficient to share trees for identical enumerators.
This fix was proposed by Jakub.

gcc/cp/ChangeLog:

	* decl.cc (finish_enum_value_list): Use fold_convert instead of
	copy_node.

Co-authored-by: Jakub Jelinek <jakub@redhat.com>
Reviewed-by: Jason Merrill <jason@redhat.com>

Author: mpolacek

gcc/cp/decl.cc

@@ -18958,13 +18958,9 @@ finish_enum_value_list (tree enumtype)
         value = perform_implicit_conversion (underlying_type,
                                              DECL_INITIAL (decl),
                                              tf_warning_or_error);
-      /* Do not clobber shared ints.  */
-      if (value != error_mark_node)
-	{
-	  value = copy_node (value);
+      /* Do not clobber shared ints.  But do share identical enumerators.  */
+      value = fold_convert (enumtype, value);
 
-	  TREE_TYPE (value) = enumtype;
-	}
       DECL_INITIAL (decl) = value;
       if (export_p)
 	DECL_MODULE_EXPORT_P (decl) = true;