GCC 5.3 build fix

It was failing with

  llvm/lib/ExecutionEngine/Orc/DebugUtils.cpp:56:10:
  error: could not convert ‘Obj’ from ‘std::unique_ptr<llvm::MemoryBuffer>’
  to ‘llvm::Expected<std::unique_ptr<llvm::MemoryBuffer> >’
     return Obj;
            ^

llvm/lib/ExecutionEngine/Orc/DebugUtils.cpp

@@ -53,7 +53,7 @@ DumpObjects::operator()(std::unique_ptr<MemoryBuffer> Obj) {
     return errorCodeToError(EC);
   DumpStream.write(Obj->getBufferStart(), Obj->getBufferSize());
 
-  return Obj;
+  return std::move(Obj);
 }
 
 StringRef DumpObjects::getBufferIdentifier(MemoryBuffer &B) {