[ADT] Make the iterator adaptor utility a touch more general by
requiring full control over the various parameters to the std::iterator
concept / trait thing. This is a precursor for adjusting these things to
where you can write a bidirectional iterator wrapping a random access
iterator with custom increment and decrement logic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207487 91177308-0d34-0410-b5e6-96231b3b80d8