Remove unused _LIBCPP_RAW_ITERATORS

[hlopko]

Summary

This change removes seemingly unused _LIBCPP_RAW_ITERATORS.

[ldionne]

LGTM, but wait a bit to see if Eric has something to say about it. I know he's been doing work in this area for a while.

[hlopko]

Thanks ldionne! Eric, what do you think?

[EricWF]

LGTM.
I can't find any usages of this internally.

PS. You don't need an LGTM from all reviewers.

[orivej]

FWIW raw pointer iterators are used at Yandex [1] and sometimes uncover issues like [2] (although othertimes they hide issues that would have been prevented by wrapped iterators).

[1] https://github.com/catboost/catboost/blob/08a65a2e/contrib/libs/cxxsupp/libcxx/include/string#L697
[2] https://reviews.llvm.org/D80473, https://reviews.llvm.org/D80475

[hlopko]

It seems that Yandex is using their own patch for the same behavior, so we can still remove it from llvm as unused? And if we want it to be a feature, it should be promoted to a flag in the config flag like other ABI-affecting features, and we should add test coverage.

[orivej]

It seems that Yandex is using their own patch for the same behavior, so we can still remove it from llvm as unused?

Yes. Yandex maintains this feature in a custom patch: https://github.com/catboost/catboost/commit/7be19209f44c05d244412dd208e80dc584d49a95

libcxx/include/string

public:
     static_assert((!is_array<value_type>::value), "Character type of basic_string must not be an array");
     static_assert(( is_standard_layout<value_type>::value), "Character type of basic_string must be standard-layout");
     static_assert(( is_trivial<value_type>::value), "Character type of basic_string must be trivial");
     static_assert(( is_same<_CharT, typename traits_type::char_type>::value),
                   "traits_type::char_type must be the same type as CharT");
     static_assert(( is_same<typename allocator_type::value_type, value_type>::value),
                   "Allocator::value_type must be same type as value_type");
 
-#if defined(_LIBCPP_RAW_ITERATORS)
-    typedef pointer                                      iterator;
-    typedef const_pointer                                const_iterator;
-#else  // defined(_LIBCPP_RAW_ITERATORS)
     typedef __wrap_iter<pointer>                         iterator;
     typedef __wrap_iter<const_pointer>                   const_iterator;
-#endif  // defined(_LIBCPP_RAW_ITERATORS)
     typedef _VSTD::reverse_iterator<iterator>             reverse_iterator;
     typedef _VSTD::reverse_iterator<const_iterator>       const_reverse_iterator;
 
 private:
 
 #ifdef _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     struct __long