Implement LWG#2908 - The less-than operator for shared pointers could do more,...

Implement LWG#2908 - The less-than operator for shared pointers could do more, and mark 2878 as complete as well (we already do that)

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@324911 91177308-0d34-0410-b5e6-96231b3b80d8