[SYCL][Test] Add regression test for sycl::vec compilation error in…

… windows, debug (#16802)

Test to isolate `sycl::vec` regression after
#14130. This PR caused `sycl::vec` to
use `std::array` as its underlying storage. However, operations on
`std::array` may emit debug-mode-only functions, on which the device
compiler may fail.

sycl/test/regression/vec_array_windows.cpp

@@ -0,0 +1,15 @@
+// Test to isolate sycl::vec regression after
+// https://github.com/intel/llvm/pull/14130. This PR caused sycl::vec to use
+// std::array as its underlying storage. However, operations on std::array
+// may emit debug-mode-only functions, on which the device compiler may fail.
+
+// REQUIRES: windows
+
+// RUN: %clangxx -fsycl -D_DEBUG %s -fsycl-device-only -Xclang -verify %s -Xclang -verify-ignore-unexpected=note,warning
+
+#include <sycl/sycl.hpp>
+
+// expected-error@* {{SYCL kernel cannot call a variadic function}}
+// expected-error@* {{SYCL kernel cannot call an undefined function without SYCL_EXTERNAL attribute}}
+// expected-error@* {{SYCL kernel cannot call an undefined function without SYCL_EXTERNAL attribute}}
+SYCL_EXTERNAL auto GetFirstElement(sycl::vec<int, 3> v) { return v[0]; }