Add a iterator and interator_range interface to the operands of a NamedMDNode.
The iterator is a little complex because we don't want to expose the implementation
details (TrackingVH) of the operand vector to clients.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203053 91177308-0d34-0410-b5e6-96231b3b80d8