cmAlgorithms: Preserve const-ness in cmRemoveDuplicates range signature

The pattern `vec.erase(cmRemoveDuplicates(vec), vec.end())` fails to
compile with GCC 4.8's libstdc++ if `cmRemoveDuplicates` returns a
`const_iterator` because `end()` returns an `iterator`.  Overload
`cmRemoveDuplicates` to return an iterator type matching the const-ness
of its argument.