IR: Add missing clone() overloads

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,...>`).

llvm-svn: 229579