[SYCL] Return shared_ptr<platform_impl> by ref (#17750)

Change the return type of getPlatformImpl() to a reference in the
context_impl and device_impl classes.

Author: slawekptak

sycl/source/detail/buffer_impl.cpp

@@ -79,7 +79,7 @@ buffer_impl::getNativeVector(backend BackendName) const {
     // doesn't have context and platform
     if (!Ctx)
       continue;
-    PlatformImplPtr Platform = Ctx->getPlatformImpl();
+    const PlatformImplPtr &Platform = Ctx->getPlatformImpl();
     assert(Platform && "Platform must be present for device context");
     if (Platform->getBackend() != BackendName)
       continue;

sycl/source/detail/context_impl.hpp

@@ -90,7 +90,7 @@ class context_impl {
   const AdapterPtr &getAdapter() const { return MPlatform->getAdapter(); }
 
   /// \return the PlatformImpl associated with this context.
-  PlatformImplPtr getPlatformImpl() const { return MPlatform; }
+  const PlatformImplPtr &getPlatformImpl() const { return MPlatform; }
 
   /// Queries this context for information.
   ///

sycl/source/detail/device_impl.hpp

@@ -290,7 +290,7 @@ class device_impl {
 
   /// @brief  Get the platform impl serving this device
   /// @return PlatformImplPtr
-  PlatformImplPtr getPlatformImpl() const { return MPlatform; }
+  const PlatformImplPtr &getPlatformImpl() const { return MPlatform; }
 
   /// Get device info string
   std::string get_device_info_string(ur_device_info_t InfoCode) const;

sycl/source/detail/usm/usm_impl.cpp

@@ -600,7 +600,8 @@ device get_pointer_device(const void *Ptr, const context &Ctxt) {
 
   // The device is not necessarily a member of the context, it could be a
   // member's descendant instead. Fetch the corresponding device from the cache.
-  std::shared_ptr<detail::platform_impl> PltImpl = CtxImpl->getPlatformImpl();
+  const std::shared_ptr<detail::platform_impl> &PltImpl =
+      CtxImpl->getPlatformImpl();
   std::shared_ptr<detail::device_impl> DevImpl =
       PltImpl->getDeviceImpl(DeviceId);
   if (DevImpl)