exchange in DelayedDestruction move-ctor

Summary: [Folly] `exchange` in `DelayedDestruction` move-ctor.

Reviewed By: knekritz

Differential Revision: D10320776

fbshipit-source-id: b6b1f910b927098e61f6f6c8e414fdfcca843033

folly/io/async/DelayedDestructionBase.h

@@ -70,9 +70,8 @@ class DelayedDestructionBase : private boost::noncopyable {
 
     DestructorGuard(const DestructorGuard& dg) : DestructorGuard(dg.dd_) {}
 
-    DestructorGuard(DestructorGuard&& dg) noexcept : dd_(dg.dd_) {
-      dg.dd_ = nullptr;
-    }
+    DestructorGuard(DestructorGuard&& dg) noexcept
+        : dd_(std::exchange(dg.dd_, nullptr)) {}
 
     DestructorGuard& operator=(DestructorGuard dg) noexcept {
       std::swap(dd_, dg.dd_);