[OpenMP] Fix build warnings

```
llvm-project/openmp/libomptarget/src/private.h:260:9: warning: 'DEBUG_PREFIX' macro redefined [-Wmacro-redefined]
#define DEBUG_PREFIX GETNAME(TARGET_NAME)
        ^
llvm-project/openmp/libomptarget/include/ompt_device_callbacks.h:22:9: note: previous definition is here
#define DEBUG_PREFIX "OMPT"
        ^
1 warning generated.
```

```
llvm-project/openmp/libomptarget/plugins-nextgen/common/PluginInterface/PluginInterface.cpp:458:14: warning: moving a local object in a return statement prevents copy elision [-Wpessimizing-move]
      return std::move(Err);
             ^
llvm-project/openmp/libomptarget/plugins-nextgen/common/PluginInterface/PluginInterface.cpp:458:14: note: remove std::move call here
      return std::move(Err);
             ^~~~~~~~~~   ~
llvm-project/openmp/libomptarget/plugins-nextgen/common/PluginInterface/PluginInterface.cpp:552:12: warning: moving a local object in a return statement prevents copy elision [-Wpessimizing-move]
    return std::move(Err);
           ^
llvm-project/openmp/libomptarget/plugins-nextgen/common/PluginInterface/PluginInterface.cpp:552:12: note: remove std::move call here
    return std::move(Err);
           ^~~~~~~~~~   ~
2 warnings generated.
```

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D154787

openmp/libomptarget/plugins-nextgen/common/PluginInterface/PluginInterface.cpp

@@ -455,7 +455,7 @@ Error GenericDeviceTy::deinit() {
 
   if (RPCHandle)
     if (auto Err = RPCHandle->deinitDevice())
-      return std::move(Err);
+      return Err;
 
   return deinitImpl();
 }
@@ -549,7 +549,7 @@ Error GenericDeviceTy::setupRPCServer(GenericPluginTy &Plugin,
     return Plugin::success();
 
   if (auto Err = Server.initDevice(*this, Plugin.getGlobalHandler(), Image))
-    return std::move(Err);
+    return Err;
 
   auto DeviceOrErr = Server.getDevice(*this);
   if (!DeviceOrErr)

openmp/libomptarget/src/private.h

@@ -257,7 +257,9 @@ struct TargetMemcpyArgsTy {
 #endif
 
 #define TARGET_NAME Libomptarget
+#ifndef DEBUG_PREFIX
 #define DEBUG_PREFIX GETNAME(TARGET_NAME)
+#endif
 
 ////////////////////////////////////////////////////////////////////////////////
 /// dump a table of all the host-target pointer pairs on failure