Constexpr check for AsyncGenerator's cleanup

Summary:
With MSVC 19.35 C++20 /O2, the check spuriously fails.
As this is a template parameter, the check can be lifted to the compile
time for the particular type. This seems to mitigate the spurious failures.

Reviewed By: Orvid, michalkielan

Differential Revision: D51098782

fbshipit-source-id: c36a1d201c6e51a19ac2a7b0737941ccb79e614d

folly/experimental/coro/AsyncGenerator.h

@@ -178,7 +178,10 @@ class FOLLY_NODISCARD AsyncGenerator {
 
   ~AsyncGenerator() {
     if (coro_) {
-      CHECK(!RequiresCleanup) << "cleanup() hasn't been called!";
+      if constexpr (RequiresCleanup) {
+        LOG(FATAL) << "cleanup() hasn't been called!";
+      }
+
       coro_.destroy();
     }
   }