[SYCL][HIP] Add interop header and devcie specialization (#7145)

Fixes #6635 for HIP.

Note that it does mean the HIP extension header must be included in user
applications in order to get the correct specialization, as is done in
the test:

```cpp
#include <sycl/ext/oneapi/backend/hip.hpp>
```

sycl/include/sycl/ext/oneapi/backend/hip.hpp

@@ -0,0 +1,31 @@
+//==--------- hip.hpp - SYCL HIP backend -----------------------------------==//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#pragma once
+
+#include <sycl/backend.hpp>
+
+namespace sycl {
+__SYCL_INLINE_VER_NAMESPACE(_V1) {
+
+template <>
+inline backend_return_t<backend::ext_oneapi_hip, device>
+get_native<backend::ext_oneapi_hip, device>(const device &Obj) {
+  // TODO use SYCL 2020 exception when implemented
+  if (Obj.get_backend() != backend::ext_oneapi_hip) {
+    throw sycl::runtime_error(errc::backend_mismatch, "Backends mismatch",
+                              PI_ERROR_INVALID_OPERATION);
+  }
+  // HIP uses a 32-bit int instead of an opaque pointer like other backends,
+  // so we need a specialization with static_cast instead of reinterpret_cast.
+  return static_cast<backend_return_t<backend::ext_oneapi_hip, device>>(
+      Obj.getNative());
+}
+
+} // __SYCL_INLINE_VER_NAMESPACE(_V1)
+} // namespace sycl

sycl/test/basic_tests/interop-hip.cpp

@@ -0,0 +1,60 @@
+// REQUIRES: hip_be
+// RUN: %clangxx %fsycl-host-only -fsyntax-only -Xclang -verify -Xclang -verify-ignore-unexpected=note %s -o %t.out
+// RUN: %clangxx %fsycl-host-only -fsyntax-only -Xclang -verify -Xclang -verify-ignore-unexpected=note -D__SYCL_INTERNAL_API %s -o %t.out
+// expected-no-diagnostics
+
+// Test for HIP interop API
+
+#include <sycl/sycl.hpp>
+#include <sycl/ext/oneapi/backend/hip.hpp>
+
+using namespace sycl;
+
+//
+// 4.5.1 SYCL application interoperability may be provided for
+// platform,
+// device,
+// context,
+// queue,
+// event,
+// buffer,
+// device_image,
+// sampled_image,
+// unsampled_image.
+
+int main() {
+
+  // Create SYCL objects
+  device Device;
+  context Context(Device);
+  queue Queue(Device);
+  event Event;
+
+  // 4.5.1.1 For each SYCL runtime class T which supports SYCL application
+  // interoperability with the SYCL backend, a specialization of return_type
+  // must be defined as the type of SYCL application interoperability native
+  // backend object associated with T for the SYCL backend, specified in the
+  // SYCL backend specification.
+  //
+  // return_type is used when retrieving the backend specific native object from
+  // a SYCL object. See the relevant backend specification for details.
+
+  backend_traits<backend::ext_oneapi_hip>::return_type<device> hip_device;
+  backend_traits<backend::ext_oneapi_hip>::return_type<context> hip_context;
+  backend_traits<backend::ext_oneapi_hip>::return_type<event> hip_event;
+  backend_traits<backend::ext_oneapi_hip>::return_type<queue> hip_queue;
+
+  // 4.5.1.2 For each SYCL runtime class T which supports SYCL application
+  // interoperability, a specialization of get_native must be defined, which
+  // takes an instance of T and returns a SYCL application interoperability
+  // native backend object associated with syclObject which can be used for SYCL
+  // application interoperability. The lifetime of the object returned are
+  // backend-defined and specified in the backend specification.
+
+  hip_device = get_native<backend::ext_oneapi_hip>(Device);
+  hip_context = get_native<backend::ext_oneapi_hip>(Context);
+  hip_event = get_native<backend::ext_oneapi_hip>(Event);
+  hip_queue = get_native<backend::ext_oneapi_hip>(Queue);
+
+  return 0;
+}