libstdc++: Implement missing P0896R4 changes to reverse_iterator [PR100639]

libstdc++: Implement missing P0896R4 changes to reverse_iterator [PR100639]

This implements the P0896R4 changes to reverse_iterator's member types
value_type, difference_type and reference in C++20 mode, which fixes
taking the reverse_iterator of an iterator with a non-integral
difference_type (such as iota_view<long long>).

libstdc++-v3/ChangeLog:

	PR libstdc++/100639
	* include/bits/stl_iterator.h (reverse_iterator::difference_type):
	In C++20 mode, define in terms of iter_difference_t as per P0896R4.
	(reverse_iterator::reference): Likewise, but with iter_reference_t.
	(reverse_iterator::value_type): Likewise, but with iter_value_t.
	* testsuite/std/ranges/adaptors/reverse.cc (test08): New test.
	* testsuite/24_iterators/reverse_iterator/100639.cc: New test.