[RDF] Use trailing return type syntax, NFC

[RDF] Use trailing return type syntax, NFC

The name rdf::Use conflicts with llvm::Use when both namespaces are
used via `using namespace`. Specifically this happened in the declaration
of DataFlowGraph::newUse (in RDFGraph.cpp):
```
using namespace rdf;

Use newUse(...);  <-- Lookup conflict for "Use"
```
Since the TRT lookup starts in a different namespace than that of the
leading type, this serves as a workaround. In general the rdf namespace
will not likely be introduced via `using namespace`, so this shouldn't
be a problem elsewhere.