c++: format attribute redeclaration [PR116954]

c++: format attribute redeclaration [PR116954]

Here when merging the two decls, remove_contract_attributes loses
ATTR_IS_DEPENDENT on the format attribute, so apply_late_template_attributes
just returns, so the attribute doesn't get propagated to the type where the
warning looks for it.

Fixed by using copy_node instead of tree_cons to preserve flags.

	PR c++/116954

gcc/cp/ChangeLog:

	* contracts.cc (remove_contract_attributes): Preserve flags
	on the attribute list.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wformat-3.C: New test.