This adds another chunk of the <ranges> header.
The changes from P1456R1 (Move-only views) and P1862R1 (Range adaptors
for non-copyable iterators) are included, but not the changes from
P1870R1 (forwarding-range<T> is too subtle).
The tests for subrange and iota_view are poor and should be improved.
* include/bits/regex.h (match_results): Specialize __enable_view_impl.
* include/bits/stl_set.h (set): Likewise.
* include/bits/unordered_set.h (unordered_set, unordered_multiset):
Likewise.
* include/debug/multiset.h (__debug::multiset): Likewise.
* include/debug/set.h (__debug::set): Likewise.
* include/debug/unordered_set (__debug::unordered_set)
(__debug::unordered_multiset): Likewise.
* include/std/ranges (ranges::view, ranges::enable_view)
(ranges::view_interface, ranges::subrange, ranges::empty_view)
(ranges::single_view, ranges::views::single, ranges::iota_view)
(ranges::views::iota): Define for C++20.
* testsuite/std/ranges/empty_view.cc: New test.
* testsuite/std/ranges/iota_view.cc: New test.
* testsuite/std/ranges/single_view.cc: New test.
* testsuite/std/ranges/view.cc: New test.
From-SVN: r278370
