libstdc++/68877 Reimplement std::__is_swappable

libstdc++/68877 Reimplement std::__is_swappable

2016-01-12  Daniel Kruegler  <daniel.kruegler@gmail.com>

	PR libstdc++/68877
	* include/std/type_traits: Following N4511, reimplement __is_swappable
	and __is_nothrow_swappable. Move __is_swappable to namespace std,
	adjust callers. Use __is_nothrow_swappable in swap.
	* include/bits/move.h: Use __is_nothrow_swappable in swap.
	* testsuite/20_util/is_nothrow_swappable/value.cc: Extend; remove
	__is_swappable related tests.
	* testsuite/20_util/is_swappable/value.cc: New.
	* testsuite/20_util/is_swappable/requirements/
	explicit_instantiation.cc: New.
	* testsuite/20_util/is_swappable/requirements/typedefs.cc: New.
	* testsuite/25_algorithms/swap/68877.cc: New.

From-SVN: r232296