[SYCL][E2E] Test recursion in device code (#15605)

Recently we allowed any C++ features to be used in manifestly
constant-evaluated expressions in device code. This is an E2E test case
testing that recursion can be safely used in this way.

Author: Fznamznon

sycl/test-e2e/Basic/recursion-in-constexpr.cpp

@@ -0,0 +1,53 @@
+// RUN: %{build} -o %t.out
+// RUN: %{run} %t.out
+
+//==---------- recursion-in-constexpr.cpp - test recursion in constexpr ----==//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+#include <iostream>
+#include <sycl/detail/core.hpp>
+
+
+unsigned long long constexpr factorial(int n) {
+  if (n == 0)
+    return 1;
+  return n * factorial(n - 1);
+}
+
+constexpr int X = 5;
+constexpr int DataLen = 5;
+
+template <int A> struct GetNTTP {
+  static const int N = A;
+};
+
+int main() {
+  sycl::queue q;
+  int res[DataLen] = {0};
+  {
+  sycl::buffer<int> buf{res, {DataLen}};
+
+  q.submit([&](sycl::handler &cgh) {
+    sycl::accessor acc{buf, cgh};
+    cgh.single_task([=] {
+      constexpr int C = factorial(X);
+      for (int i = 0; i < DataLen; ++i)
+        acc[i] = C;
+      acc[DataLen - 1] = GetNTTP<factorial(X)>::N;
+    });
+  });
+  }
+
+  for (int i = 0; i < DataLen; ++i) {
+    if (res[i] != factorial(X)) {
+      std::cout << "FAIL " << std::endl;
+      return -1;
+    }
+  }
+
+  return 0;
+}