LWG issue #2188: Reverse iterator does not fully support targets that overload operator&. Also mark #2272 and #2299  as complete; libc++ already implements them.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@202339 91177308-0d34-0410-b5e6-96231b3b80d8