Apply C++20 changes to various iterator types

Apply C++20 changes to various iterator types

This ensures that __normal_iterator<T*, C> satisfies the
contiguous_iterator concept, by defining the iterator_concept member
type.

Also update vector<bool>'s iterators, reverse_iterator,
istreambuf_iterator and ostreambuf_iterator to meet the C++20
requirements.

	PR libstdc++/92272
	* include/bits/stl_bvector.h (_Bit_iterator::pointer)
	(_Bit_const_iterator::pointer): Define as void for C++20.
	* include/bits/stl_iterator.h (reverse_iterator::operator->()): Add
	constraints for C++20.
	(__normal_iterator::iterator_concept): Define for C++20.
	* include/bits/streambuf_iterator.h (istreambuf_iterator::pointer):
	Define as void for C++20.
	(ostreambuf_iterator::difference_type): Define as ptrdiff_t for C++20.
	(ostreambuf_iterator::ostreambuf_iterator()): Add default constructor
	for C++20.
	* testsuite/23_containers/vector/bool/iterator_c++20.cc: New test.
	* testsuite/24_iterators/bidirectional/concept.cc: New test.
	* testsuite/24_iterators/bidirectional/tag.cc: New test.
	* testsuite/24_iterators/contiguous/concept.cc: New test.
	* testsuite/24_iterators/contiguous/tag.cc: New test.
	* testsuite/24_iterators/forward/concept.cc: New test.
	* testsuite/24_iterators/forward/tag.cc: New test.
	* testsuite/24_iterators/input/concept.cc: New test.
	* testsuite/24_iterators/input/tag.cc: New test.
	* testsuite/24_iterators/istreambuf_iterator/requirements/typedefs.cc:
	New test.
	* testsuite/24_iterators/ostreambuf_iterator/requirements/typedefs.cc:
	New test.
	* testsuite/24_iterators/output/concept.cc: New test.
	* testsuite/24_iterators/output/tag.cc: New test.
	* testsuite/24_iterators/random_access/concept.cc: New test.
	* testsuite/24_iterators/random_access/tag.cc: New test.
	* testsuite/24_iterators/range_operations/advance_debug_neg.cc: New
	test.
	* testsuite/24_iterators/random_access_iterator/26020.cc: Move to ...
	* testsuite/24_iterators/operations/26020.cc: ... here.
	* testsuite/24_iterators/random_access_iterator/
	string_vector_iterators.cc: Move to ...
	* testsuite/24_iterators/random_access/string_vector_iterators.cc: ...
	here.

From-SVN: r277629