The standard requires that the std::thread constructor is constrained so
it can't be called with a first argument of type std::thread. The
current implementation only meets that requirement if the constructor is
called with one argument, by using deleted overloads. This uses an
enable_if constraint to enforce the requirement for any number of
arguments.

Also add a static assertion to give a more readable error for invalid
arguments that cannot be invoked. Also simplify _Invoker to reduce the
error cascade for ill-formed instantiations with non-invocable
arguments.

	PR libstdc++/84535
	* include/std/thread (thread::__not_same): New SFINAE helper.
	(thread::thread(_Callable&&, _Args&&...)): Add SFINAE constraint that
	first argument is not a std::thread. Add static assertion to check
	INVOKE expression is valid.
	(thread::thread(thread&), thread::thread(const thread&&)): Remove.
	(thread::_Invoke::_M_invoke, thread::_Invoke::operator()): Use
	__invoke_result for return types and remove exception specifications.
	* testsuite/30_threads/thread/cons/84535.cc: New.

From-SVN: r259893