'noexcept' on the 'detach' fns called from Future/Promise dtors

Summary: Core::detachFuture() & detachPromise() are called from dtors; make them 'noexcept'

Reviewed By: yfeldblum

Differential Revision: D7995163

fbshipit-source-id: 5c15e0516be2559a2ce1d283dc6fa75c2f506cd8

folly/futures/detail/Core.h

@@ -201,12 +201,12 @@ class Core final {
   }
 
   /// Called by a destructing Future (in the Future thread, by definition)
-  void detachFuture() {
+  void detachFuture() noexcept {
     detachOne();
   }
 
   /// Called by a destructing Promise (in the Promise thread, by definition)
-  void detachPromise() {
+  void detachPromise() noexcept {
     DCHECK(result_);
     detachOne();
   }