Add missing specialized node overloads for `MDNode::clone()` (they were
on most of the node types already, but missing from the others).
`MDNode::clone()` returns `TempMDNode` (`std::unique_ptr<MDNode,...>`),
while `TempMDSubrange::clone()` (for example) returns the more
convenient `TempMDSubrange` (`std::unique_ptr<TempMDSubrange,...>`).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229579 91177308-0d34-0410-b5e6-96231b3b80d8
15aa8bae80