[SYCL] Fix ABI break after "Refactor HandlerAccess::postProcess" (#19502)

#19203 changed current reduction
implementation but the old one was inlined into user applications and it
relied on the `handler::MIsFinalized` hacky behavior. As such, we have
to keep it in non-preview mode.

Author: aelovikov-intel

sycl/source/handler.cpp

@@ -432,6 +432,28 @@ detail::EventImplPtr handler::finalize() {
 #else
 event handler::finalize() {
 #endif
+#ifndef __INTEL_PREVIEW_BREAKING_CHANGES
+  // Old reduction implementation, prior to
+  //   https://github.com/intel/llvm/pull/18794
+  //   https://github.com/intel/llvm/pull/18898
+  //   https://github.com/intel/llvm/pull/19203
+  // relied on explicit calls to handler::finalize and those calls were inlined
+  // into the user applications. As such, we have to preserve the following
+  // behavior for ABI-compatibility purposes:
+  if (MIsFinalizedDoNotUse)
+    return MLastEventDoNotUse;
+
+  MIsFinalizedDoNotUse = true;
+  // Use macros to trick clang-format:
+#define WRAP_BODY_BEGIN MLastEventDoNotUse = [this]() {
+#define WRAP_BODY_END                                                          \
+  }                                                                            \
+  ();                                                                          \
+  return MLastEventDoNotUse;
+
+  WRAP_BODY_BEGIN
+#endif
+
   const auto &type = getType();
   detail::queue_impl *Queue = impl->get_queue_or_null();
   ext::oneapi::experimental::detail::graph_impl *Graph =
@@ -937,6 +959,13 @@ event handler::finalize() {
 #else
   return detail::createSyclObjFromImpl<event>(Event);
 #endif
+
+#ifndef __INTEL_PREVIEW_BREAKING_CHANGES
+  WRAP_BODY_END
+
+#undef WRAP_BODY_BEGIN
+#undef WRAP_BODY_END
+#endif
 }
 
 void handler::addReduction(const std::shared_ptr<const void> &ReduObj) {