3a200dee has a logic bug where we tried to conform C++ iterator types to `UnsafeCxxContiguousIterator` protocol based on their nested type called `iterator_category`. The C++20 standard says we should rely on `iterator_concept` instead.
https://en.cppreference.com/w/cpp/iterator/iterator_tags#Iterator_concept
Despite what the name suggests, we are not actually using C++ concepts in this change.
rdar://137877849
This adds a pair of Swift protocols that represents C++ iterator types conforming to `std::contiguous_iterator_tag` requirements. These are random access iterators that guarantee that the values are stored in consequent memory addresses.
This will be used to optimize usage of C++ containers such as `std::vector` from Swift, for instance, by providing an overload of `withContiguousStorageIfAvailable` for contiguous containers.
rdar://137877849
