Implement LWG 2360: 'reverse_iterator::operator*() is unimplementable'. Note...

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.

llvm-svn: 203587