Folly coro: avoid KeepAlive copy in promise

Summary: This prevents one acquire/release.

Reviewed By: lewissbaker

Differential Revision: D20924353

fbshipit-source-id: 8c5135bd2ebc03b736b1ea3dbb385e01218cffbc

folly/experimental/coro/ViaIfAsync.h

@@ -40,7 +40,8 @@ class ViaCoroutine {
  public:
   class promise_type {
    public:
-    promise_type(folly::Executor::KeepAlive<> executor) noexcept
+    // Passed as lvalue by compiler, but should have no other dependencies
+    promise_type(folly::Executor::KeepAlive<>& executor) noexcept
         : executor_(std::move(executor)) {}
 
     ViaCoroutine get_return_object() noexcept {