[libcxx] deprecates/removes `std::raw_storage_iterator`

C++17 deprecates `std::raw_storage_iterator` and C++20 removes it.

Implements part of:
  * P0174R2 'Deprecating Vestigial Library Parts in C++17'
  * P0619R4 'Reviewing Deprecated Facilities of C++17 for C++20'

Differential Revision: https://reviews.llvm.org/D101730