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
