Creating debug information via llvm-c

I'm experimenting with adding debug information to Rust, and the
codegen wrap llvm-c. I believe that I'm able to create metadata nodes
via LLVMMDNode, and it looks like I can add them to instructions as
operands via LLVMSetMetadata, but I don't see any way to create
free-floating named metadata nodes (such as, or even
nodes with unique numeric IDs. Am I missing something? We're currently
working off of rev 142082.