The std::thread constructor creates (and then moves) an unnecessary
temporary copy of each argument. Optimize it to only make the one copy
that is required.

	PR libstdc++/69724
	* include/std/thread (thread::_State_impl, thread::_S_make_state):
	Replace single _Callable parameter with variadic _Args pack, to
	forward them directly to the tuple of decayed copies.
	* testsuite/30_threads/thread/cons/69724.cc: New test.

From-SVN: r271166