[libomptarget] Remove `Devices` from `RTLInfoTy`

[sdmitriev]

Summary

This patch removes RTLInfoTy::Devices to fix a potential problem of using invalid pointers stored in this vector. The problem is described in comments to this patch https://reviews.llvm.org/D51623

[Hahnfeld]

LG, but please revise the summary to say that the variable was unused. I think that's enough of motivation, no need to refer to "potential problems" or other patches.

[sdmitriev]

Ok.

libomptarget/src/rtl.h

struct RTLInfoTy {
   typedef int32_t(run_team_region_ty)(int32_t, void *, void **, ptrdiff_t *,
                                       int32_t, int32_t, int32_t, uint64_t);
 
   int32_t Idx;                     // RTL index, index is the number of devices
                                    // of other RTLs that were registered before,
                                    // i.e. the OpenMP index of the first device
                                    // to be registered with this RTL.
   int32_t NumberOfDevices;         // Number of devices this RTL deals with.
-  std::vector<DeviceTy *> Devices; // one per device (NumberOfDevices in total).
 
   void *LibraryHandler;
 
 #ifdef OMPTARGET_DEBUG
   std::string RTLName;
 #endif
 
   // Functions implemented in the RTL.
#endif
   // Mutex for thread-safety when calling RTL interface functions.
   // It is easier to enforce thread-safety at the libomptarget level,
   // so that developers of new RTLs do not have to worry about it.
   std::mutex Mtx;
 
   // The existence of the mutex above makes RTLInfoTy non-copyable.
   // We need to provide a copy constructor explicitly.
   RTLInfoTy()
-      : Idx(-1), NumberOfDevices(-1), Devices(), LibraryHandler(0),
+      : Idx(-1), NumberOfDevices(-1), LibraryHandler(0),
 #ifdef OMPTARGET_DEBUG
         RTLName(),
 #endif
         is_valid_binary(0), number_of_devices(0), init_device(0),
         load_binary(0), data_alloc(0), data_submit(0), data_retrieve(0),
         data_delete(0), run_region(0), run_team_region(0), isUsed(false),
         Mtx() {}
 
   RTLInfoTy(const RTLInfoTy &r) : Mtx() {
     Idx = r.Idx;
     NumberOfDevices = r.NumberOfDevices;
-    Devices = r.Devices;
     LibraryHandler = r.LibraryHandler;
 #ifdef OMPTARGET_DEBUG
     RTLName = r.RTLName;
 #endif
     is_valid_binary = r.is_valid_binary;
     number_of_devices = r.number_of_devices;
     init_device = r.init_device;
     load_binary = r.load_binary;

libomptarget/src/rtl.cpp

for (auto &R : RTLs.AllRTLs) {
         size_t start = Devices.size();
         Devices.resize(start + R.NumberOfDevices, device);
         for (int32_t device_id = 0; device_id < R.NumberOfDevices;
             device_id++) {
           // global device ID
           Devices[start + device_id].DeviceID = start + device_id;
           // RTL local device ID
           Devices[start + device_id].RTLDeviceID = device_id;
-
-          // Save pointer to device in RTL in case we want to unregister the RTL
-          R.Devices.push_back(&Devices[start + device_id]);
         }
 
         // Initialize the index of this RTL and save it in the used RTLs.
         R.Idx = (RTLs.UsedRTLs.empty())
                     ? 0
                     : RTLs.UsedRTLs.back()->Idx +
                           RTLs.UsedRTLs.back()->NumberOfDevices;
         assert((size_t) R.Idx == start &&