Implement the last bit of P0031: 'A Proposal to Add Constexpr Modifiers to reverse_iterator, move_iterator, array and Range Access' for C++17

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290976 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed