re PR libstdc++/60564 ([C++11] The std::packaged_task constructor taking a reference to a functor does not copy its argument.)

	PR libstdc++/60564
	* include/std/future (__future_base::_Task_state<>): Change
	constructors to template functions using perfect forwarding.
	(__create_task_state): Use decayed type as stored task.
	(packaged_task::packaged_task(_Fn&&)): Forward instead of moving.
	* testsuite/30_threads/packaged_task/60564.cc: New.

From-SVN: r208656