[LLJIT] Add std::move() as a workaround for older compilers

Clang 3.8 isn't able to bind the variable to rvalue-ref which breaks the build.

llvm/lib/ExecutionEngine/Orc/LLJIT.cpp

@@ -928,7 +928,7 @@ Error LLJITBuilderState::prepareForConstruction() {
             ES, std::make_unique<jitlink::InProcessMemoryManager>());
         ObjLinkingLayer->addPlugin(std::make_unique<EHFrameRegistrationPlugin>(
             jitlink::InProcessEHFrameRegistrar::getInstance()));
-        return ObjLinkingLayer;
+        return std::move(ObjLinkingLayer);
       };
     }
   }