[SYCL][NFC] Fix deprecation note for mem_fence (#4198)

Author: Alexander Batashev

sycl/include/CL/sycl/nd_item.hpp

@@ -122,7 +122,7 @@ template <int dimensions = 1> class nd_item {
   /// Executes a work-group mem-fence with memory ordering on the local address
   /// space, global address space or both based on the value of \p accessSpace.
   template <access::mode accessMode = access::mode::read_write>
-  __SYCL2020_DEPRECATED("use sycl::group_barrier() free function instead")
+  __SYCL2020_DEPRECATED("use sycl::atomic_fence() free function instead")
   void mem_fence(
       typename detail::enable_if_t<accessMode == access::mode::read ||
                                        accessMode == access::mode::write ||

sycl/test/warnings/sycl_2020_deprecations.cpp

@@ -116,8 +116,8 @@ int main() {
     CGH.parallel_for<class Test>(
         sycl::nd_range<1>{sycl::range{10}, sycl::range{10}, sycl::range{1}},
         [](sycl::nd_item<1> it) {
-          // expected-warning@+2{{'mem_fence' is deprecated: use sycl::group_barrier() free function instead}}
-          // expected-warning@+1{{'mem_fence<sycl::access::mode::read_write>' is deprecated: use sycl::group_barrier() free function instead}}
+          // expected-warning@+2{{'mem_fence' is deprecated: use sycl::atomic_fence() free function instead}}
+          // expected-warning@+1{{'mem_fence<sycl::access::mode::read_write>' is deprecated: use sycl::atomic_fence() free function instead}}
           it.mem_fence();
         });
   });