libstdc++: Implement LWG 3798 for range adaptors [PR106676]

LWG 3798 modified the iterator_category of the iterator types for
transform_view, join_with_view, zip_transform_view and
adjacent_transform_view, to allow the iterator's reference type to be an
rvalue reference.

libstdc++-v3/ChangeLog:

	PR libstdc++/106676
	* include/bits/iterator_concepts.h (__cpp17_fwd_iterator): Use
	is_reference instead of is_value_reference.
	rvalue references.
	* include/std/ranges (transform_view:__iter_cat::_S_iter_cat):
	Likewise.
	(zip_transform_view::__iter_cat::_S_iter_cat): Likewise.
	(adjacent_transform_view::__iter_cat::_S_iter_cat): Likewise.
	(join_with_view::__iter_cat::_S_iter_cat): Likewise.
	* testsuite/std/ranges/adaptors/transform.cc: Check
	iterator_category when the transformation function returns an
	rvalue reference type.

Reviewed-by: Patrick Palka <ppalka@redhat.com>