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

[Louis Dionne via cfe-commits]

================
@@ -0,0 +1,41 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// This test will fail if the number of devices detected by OpenMP is larger
+// than zero but for_each is not executed on the device.
+
+// UNSUPPORTED: c++03, c++11, c++14, gcc
+
+// ADDITIONAL_COMPILE_FLAGS: -O2 -Wno-pass-failed -fopenmp --offload-arch=native
+
+// REQUIRES: openmp_pstl_backend
+
+#include <algorithm>
+#include <cassert>
+#include <execution>
+#include <vector>
+#include <omp.h>
+
+int main(void) {
+  // We only run the test if a device is detected by OpenMP
+  if (omp_get_num_devices() < 1)
+    return 0;
+
+  // Initializing test array
+  const int __test_size = 10000;
+  std::vector<int> __v(__test_size);
+  std::for_each(std::execution::par_unseq, __v.begin(), __v.end(), [](int& n) {
+    // Returns true if executed on the host
+    n = omp_is_initial_device();
+  });
+
+  auto __idx = std::find_if(std::execution::par_unseq, __v.begin(), __v.end(), [](int& n) -> bool { return n > 0; });
----------------
ldionne wrote:

This should be just

```
for (int& n : v) {
  assert(n > 0);
}
```

This isn't part of the PSTL code we're trying to test, so no reason to use PSTL here.

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