replace [[nodiscard] with FOLLY_NODISCARD

Summary: Correct use of portability macro.

Reviewed By: yfeldblum, manderyao

Differential Revision: D20520392

fbshipit-source-id: b21fb13aa7a523db21a506b71e65047edbb17684

folly/Executor.h

@@ -308,7 +308,7 @@ class BlockingGuard;
 BlockingGuard makeBlockingDisallowedGuard(folly::StringPiece executorName);
 BlockingGuard makeBlockingAllowedGuard();
 
-class [[nodiscard]] BlockingGuard {
+class FOLLY_NODISCARD BlockingGuard {
  public:
   ~BlockingGuard();