libstdc++: Add [[nodiscard]] to iterators and related utilities

libstdc++: Add [[nodiscard]] to iterators and related utilities

This adds [[nodiscard]] throughout <iterator>, as proposed by P2377R0
(with some minor corrections).

The attribute is added for all modes from C++11 up, using
[[__nodiscard__]] or _GLIBCXX_NODISCARD where C++17 [[nodiscard]] can't
be used directly.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/iterator_concepts.h (iter_move): Add
	[[nodiscard]].
	* include/bits/range_access.h (begin, end, cbegin, cend)
	(rbegin, rend, crbegin, crend, size, data, ssize): Likewise.
	* include/bits/ranges_base.h (ranges::begin, ranges::end)
	(ranges::cbegin, ranges::cend, ranges::rbegin, ranges::rend)
	(ranges::crbegin, ranges::crend, ranges::size, ranges::ssize)
	(ranges::empty, ranges::data, ranges::cdata): Likewise.
	* include/bits/stl_iterator.h (reverse_iterator, __normal_iterator)
	(back_insert_iterator, front_insert_iterator, insert_iterator)
	(move_iterator, move_sentinel, common_iterator)
	(counted_iterator): Likewise.
	* include/bits/stl_iterator_base_funcs.h (distance, next, prev):
	Likewise.
	* include/bits/stream_iterator.h (istream_iterator)
	(ostream_iterartor): Likewise.
	* include/bits/streambuf_iterator.h (istreambuf_iterator)
	(ostreambuf_iterator): Likewise.
	* include/std/ranges (views::single, views::iota, views::all)
	(views::filter, views::transform, views::take, views::take_while)
	(views::drop, views::drop_while, views::join, views::lazy_split)
	(views::split, views::counted, views::common, views::reverse)
	(views::elements): Likewise.
	* testsuite/20_util/rel_ops.cc: Use -Wno-unused-result.
	* testsuite/24_iterators/move_iterator/greedy_ops.cc: Likewise.
	* testsuite/24_iterators/normal_iterator/greedy_ops.cc:
	Likewise.
	* testsuite/24_iterators/reverse_iterator/2.cc: Likewise.
	* testsuite/24_iterators/reverse_iterator/greedy_ops.cc:
	Likewise.
	* testsuite/21_strings/basic_string/range_access/char/1.cc:
	Cast result to void.
	* testsuite/21_strings/basic_string/range_access/wchar_t/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/range_access/char/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/range_access/wchar_t/1.cc:
	Likewise.
	* testsuite/23_containers/array/range_access.cc: Likewise.
	* testsuite/23_containers/deque/range_access.cc: Likewise.
	* testsuite/23_containers/forward_list/range_access.cc:
	Likewise.
	* testsuite/23_containers/list/range_access.cc: Likewise.
	* testsuite/23_containers/map/range_access.cc: Likewise.
	* testsuite/23_containers/multimap/range_access.cc: Likewise.
	* testsuite/23_containers/multiset/range_access.cc: Likewise.
	* testsuite/23_containers/set/range_access.cc: Likewise.
	* testsuite/23_containers/unordered_map/range_access.cc:
	Likewise.
	* testsuite/23_containers/unordered_multimap/range_access.cc:
	Likewise.
	* testsuite/23_containers/unordered_multiset/range_access.cc:
	Likewise.
	* testsuite/23_containers/unordered_set/range_access.cc:
	Likewise.
	* testsuite/23_containers/vector/range_access.cc: Likewise.
	* testsuite/24_iterators/customization_points/iter_move.cc:
	Likewise.
	* testsuite/24_iterators/istream_iterator/sentinel.cc:
	Likewise.
	* testsuite/24_iterators/istreambuf_iterator/sentinel.cc:
	Likewise.
	* testsuite/24_iterators/move_iterator/dr2061.cc: Likewise.
	* testsuite/24_iterators/operations/prev_neg.cc: Likewise.
	* testsuite/24_iterators/ostreambuf_iterator/2.cc: Likewise.
	* testsuite/24_iterators/range_access/range_access.cc:
	Likewise.
	* testsuite/24_iterators/range_operations/100768.cc: Likewise.
	* testsuite/26_numerics/valarray/range_access2.cc: Likewise.
	* testsuite/28_regex/range_access.cc: Likewise.
	* testsuite/experimental/string_view/range_access/char/1.cc:
	Likewise.
	* testsuite/experimental/string_view/range_access/wchar_t/1.cc:
	Likewise.
	* testsuite/ext/vstring/range_access.cc: Likewise.
	* testsuite/std/ranges/adaptors/take.cc: Likewise.
	* testsuite/std/ranges/p2259.cc: Likewise.