Opportunity to use exchange in ScopeGuardImpl

Summary: [Folly] Opportunity to use `exchange` in `ScopeGuardImpl`.

Reviewed By: Orvid

Differential Revision: D10119838

fbshipit-source-id: 64a00f4f60d913f8af248ebf891f730d2c69fec8

folly/ScopeGuard.h

@@ -25,6 +25,7 @@
 
 #include <folly/Portability.h>
 #include <folly/Preprocessor.h>
+#include <folly/Utility.h>
 #include <folly/lang/UncaughtExceptions.h>
 
 namespace folly {
@@ -88,8 +89,7 @@ class ScopeGuardImpl : public ScopeGuardImplBase {
     // on the value of other.dismissed_. The following lines only execute
     // if the move/copy succeeded, in which case *this assumes ownership of
     // the cleanup action and dismisses other.
-    dismissed_ = other.dismissed_;
-    other.dismissed_ = true;
+    dismissed_ = exchange(other.dismissed_, true);
   }
 
   ~ScopeGuardImpl() noexcept(InvokeNoexcept) {