We are forwarding the second argument of views::iota using the wrong type,
causing compile errors when calling views::iota with a value and bound of
different types, like in the test case below.

libstdc++-v3/ChangeLog:

	* include/std/ranges (iota_view): Forward declare _Sentinel.
	(iota_view::_Iterator): Befriend _Sentinel.
	(iota_view::_Sentinel::_M_equal): New member function.
	(iota_view::_Sentinel::operator==): Use it.
	(views::_Iota::operator()): Forward __f using the correct type.
	* testsuite/std/ranges/access/ssize.cc (test06): Don't call views::iota
	with integers of different signedness, to appease iota_view's deduction
	guide.
	* testsuite/std/ranges/iota/iota_view.cc: Augment test.