The converting constructor of join_view::_Sentinel<true> needs to be able to
access the private members of join_view::_Sentinel<false>.

libstdc++-v3/ChangeLog:

	* include/std/ranges (join_view::_Sentinel<_Const>): Befriend
	join_view::_Sentinel<!_Const>.
	* testsuite/std/ranges/adaptors/join.cc: Augment test.