libstdc++: Fix up lambda in join_view::_Iterator::operator++ [PR100290]

libstdc++: Fix up lambda in join_view::_Iterator::operator++ [PR100290]

Currently, the return type of this lambda is decltype(auto), so the
lambda ends up returning a copy of _M_parent->_M_inner rather than a
reference to it when _S_ref_glvalue is false.  This means _M_inner and
ranges::end(__inner_range) are respectively an iterator and sentinel for
different ranges, so comparing them is undefined.

libstdc++-v3/ChangeLog:

	PR libstdc++/100290
	* include/std/ranges (join_view::_Iterator::operator++): Correct
	the return type of the lambda to avoid returning a copy of
	_M_parent->_M_inner.
	* testsuite/std/ranges/adaptors/join.cc (test10): New test.