[ADT] Factor out the facade aspect of the iterator_adaptor_base into its
own CRTP base class for more general purpose use. Add some clarifying
comments for the exact way in which the adaptor uses it. Hopefully this
will help us write increasingly full featured iterators. This is
becoming important as they start to be used heavily inside of ranges.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207072 91177308-0d34-0410-b5e6-96231b3b80d8