[SYCL] Add LIT regression test for ambiguous call to memcpy (#14995)

`detail::memcpy`, even though in a different namespace, can cause
ambiguity with libc's memcpy, due to argument dependent lookup (ADL).
This compile-only
test checks for ambiguity in call to `detail::memcpy`.

As discussed in
#14836 (comment)

---------

Co-authored-by: Steffen Larsen <steffen.larsen@intel.com>

Author: uditagarwal97

sycl/test/regression/ambigious-memcpy.cpp

@@ -0,0 +1,17 @@
+// RUN: %clang -fsycl -fsycl-device-only -fsyntax-only %s
+
+// detail::memcpy, even though in a different namespace, can cause ambiguity
+// with libc's memcpy, due to argument dependent lookup (ADL). This compile-only
+// test checks for ambiguity in call to detail::memcpy.
+
+#include <sycl/vector.hpp>
+
+template <typename T> void foo(T *dst, T *src, size_t count) {
+  memcpy(dst, src, count * sizeof(T));
+}
+
+using T = sycl::vec<int, 1>;
+
+SYCL_EXTERNAL void bar(T *dst, T *src, size_t count) {
+  foo(dst, src, count * sizeof(T));
+}