libstdc++: Remove redundant copying of std::async arguments [PR 69724]

libstdc++: Remove redundant copying of std::async arguments [PR 69724]

As was previously done for std::thread, this removes an unnecessary copy
of an rvalue of type thread::_Invoker. Instead of creating the rvalue
and then moving that into the shared state, the member of the shared
state is initialized directly from the forwarded callable and bound
arguments.

This also slightly simplifies std::thread creation to remove the
_S_make_state helper function.

libstdc++-v3/ChangeLog:

	PR libstdc++/69724
	* include/std/future (__future_base::_S_make_deferred_state)
	(__future_base::_S_make_async_state): Remove.
	(__future_base::_Deferred_state): Change constructor to accept a
	parameter pack of arguments and forward them to the call
	wrapper.
	(__future_base::_Async_state_impl): Likewise. Replace lambda
	expression with a named member function.
	(async): Construct state object directly from the arguments,
	instead of using thread::__make_invoker, _S_make_deferred_state
	and _S_make_async_state. Move shared state into the returned
	future.
	* include/std/thread (thread::_Call_wrapper): New alias
	template for use by constructor and std::async.
	(thread::thread(Callable&&, Args&&...)): Create state object
	directly instead of using _S_make_state.
	(thread::__make_invoker, thread::__decayed_tuple)
	(thread::_S_make_state): Remove.
	* testsuite/30_threads/async/69724.cc: New test.