libstdc++: Fix ranges::join_view::_Iterator::operator-> [LWG 3500]

libstdc++: Fix ranges::join_view::_Iterator::operator-> [LWG 3500]

This applies the proposed resolution of LWG 3500, which corrects the
return type and constraints of this member function to use the right
iterator type.  Additionally, a nearby local variable is uglified.

libstdc++-v3/ChangeLog:

	* include/std/ranges (join_view::_Iterator::_M_satisfy): Uglify
	local variable inner.
	(join_view::_Iterator::operator->): Use _Inner_iter instead of
	_Outer_iter in the function signature as per LWG 3500.
	* testsuite/std/ranges/adaptors/join.cc (test08): Test it.