[SYCL] Deprecate sycl::atomics in SYCL 2020 mode (#5440)

Explicitly marked class cl::sycl::atomic as deprecated to meet an ad hoc requirement of new SYCL standard.

Author: IgorKharchikov

sycl/include/CL/sycl/atomic.hpp

@@ -168,7 +168,8 @@ namespace sycl {
 
 template <typename T, access::address_space addressSpace =
                           access::address_space::global_space>
-class atomic {
+class __SYCL2020_DEPRECATED(
+    "sycl::atomic is deprecated since SYCL 2020") atomic {
   friend class atomic<T, access::address_space::global_space>;
   static_assert(detail::IsValidAtomicType<T>::value,
                 "Invalid SYCL atomic type. Valid types are: int, "

sycl/test/warnings/sycl_2020_deprecations.cpp

@@ -1,7 +1,4 @@
 // RUN: %clangxx %fsycl-host-only -fsyntax-only -sycl-std=2020 -Xclang -verify -Xclang -verify-ignore-unexpected=note %s -o %t.out
-// RUN: %clangxx %fsycl-host-only -fsyntax-only -Xclang -verify -Xclang -verify-ignore-unexpected=note %s -o %t.out
-// RUN: %clangxx %fsycl-host-only -fsyntax-only -sycl-std=2017 -Xclang -verify -Xclang -verify-ignore-unexpected=note %s -o %t.out
-// RUN: %clangxx %fsycl-host-only -fsyntax-only -sycl-std=1.2.1 -Xclang -verify -Xclang -verify-ignore-unexpected=note %s -o %t.out
 
 #include <CL/sycl.hpp>
 #include <sycl/ext/intel/online_compiler.hpp>
@@ -176,6 +173,11 @@ int main() {
 
   // expected-warning@+1{{'barrier' is deprecated: use 'ext_oneapi_barrier' instead}}
   Queue.submit([&](sycl::handler &CGH) { CGH.barrier(); });
-  
+
+  cl::sycl::multi_ptr<int, cl::sycl::access::address_space::global_space> a(
+      nullptr);
+  // expected-warning@+1 {{'atomic<int, sycl::access::address_space::global_space>' is deprecated: sycl::atomic is deprecated since SYCL 2020}}
+  cl::sycl::atomic<int> b(a);
+
   return 0;
 }