Implement LWG 2360: 'reverse_iterator::operator*() is unimplementable'. Note that this is a (small) behavior change in the library. Reverse iterators whose base iterators' operator* return references to 'within themselves' have been sacrificed to the greater goal of avoiding data races.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@203587 91177308-0d34-0410-b5e6-96231b3b80d8