[llvm] Adding Separate OpenMP Offloading Backend to `libcxx/include/__algorithm/pstl_backends` (PR #66968)

[Louis Dionne via llvm-commits]

================
@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ALGORITHM_PSTL_BACKENDS_OPENMP_BACKEND_FOR_EACH_H
+#define _LIBCPP___ALGORITHM_PSTL_BACKENDS_OPENMP_BACKEND_FOR_EACH_H
+
+#include <__algorithm/for_each.h>
+#include <__algorithm/pstl_backends/cpu_backends/backend.h>
+#include <__algorithm/pstl_backends/openmp/backend.h>
+#include <__config>
+#include <__type_traits/is_execution_policy.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_PSTL) && _LIBCPP_STD_VER >= 17
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _ExecutionPolicy, class _ForwardIterator, class _Functor>
+_LIBCPP_HIDE_FROM_ABI void
+__pstl_for_each(__omp_backend_tag, _ForwardIterator __first, _ForwardIterator __last, _Functor __func) {
+  // It is only safe to execute for_each on the GPU, it the execution policy is
+  // parallel unsequenced, as it is the only execution policy prohibiting throwing
+  // exceptions and allowing SIMD instructions
+  if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
----------------
ldionne wrote:

Do we really want to implement this as `__parallel_for_simd_1` when it is either `par_unseq` or `unseq`, or only when it is `par_unseq`? If we want to constrain this to `par_unseq`, we should use

```
__is_unsequenced_execution_policy_v && __is_parallel_execution_policy_v
```



https://github.com/llvm/llvm-project/pull/66968