Use std::atomic_ref when available

Summary: Fixes broken builds where C++20 `std::atomic_ref` is not available.

Reviewed By: jbower-fb

Differential Revision: D94774027

fbshipit-source-id: 3ccdff4c7c1f5fb2e445a592c3868b131d7df2d8

Author: yoney

cinderx/Common/util.h

@@ -488,7 +488,12 @@ class FrozenList {
 inline void setVectorcall(
     BorrowedRef<PyFunctionObject> func,
     vectorcallfunc entry) {
+#ifdef __cpp_lib_atomic_ref
+  std::atomic_ref<vectorcallfunc>(func->vectorcall)
+      .store(entry, std::memory_order_relaxed);
+#else
   __atomic_store_n(&func->vectorcall, entry, __ATOMIC_RELAXED);
+#endif
 }
 
 using FuncVisitor = void (*)(BorrowedRef<PyFunctionObject>);