ADT: Implement MutableArrayRef::reverse_iterator

This adds rbegin/rend methods to MutableArrayRef, they will be used by a
follow-on commit in clang.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196768 91177308-0d34-0410-b5e6-96231b3b80d8