Make std::mem_fn work with varargs functions.

Make std::mem_fn work with varargs functions.

	* include/std/functional (_Mem_fn_traits): Add partial specializations
	for varargs functions.
	(_Mem_fn_base): Do not check arguments are convertible for varargs.
	(_Bind_check_arity): Add partial specializations for varargs functions.
	* include/std/type_traits (__bool_constant): Add alias template.
	* testsuite/20_util/bind/ref_neg.cc: Adjust dg-error.
	* testsuite/20_util/bind/refqual.cc: New, test ref-qualifiers.
	* testsuite/20_util/declval/requirements/1_neg.cc: Adjust dg-error.
	* testsuite/20_util/function_objects/mem_fn/refqual.cc: Test varargs.
	* testsuite/20_util/make_signed/requirements/typedefs_neg.cc: Adjust
	dg-error.
	* testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc: Adjust
	dg-error.

From-SVN: r217393