IR: MDNode => Value: NamedMDNode::addOperand()

Change `NamedMDNode::addOperand()` to take a `Value *` instead of an
`MDNode *`.  This is part of PR21433.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221359 91177308-0d34-0410-b5e6-96231b3b80d8