add attr to co_awaitTry

Summary:
This diff is actually a missing part of D67299322, which is landed omitting this change.

Considering that `co_awaitTry` is similar to `co_nothrow`, this should be safe. This change has been ready before the break but I didn't land it due to code freeze/safety concerns.

Reviewed By: yfeldblum, skrueger

Differential Revision: D68108268

fbshipit-source-id: 1157448ec1840b089509a242630fe78cf21385cb

Author: yuxuanchen1997

folly/coro/ViaIfAsync.h

@@ -750,15 +750,15 @@ template <
     typename Awaitable,
     std::enable_if_t<!is_must_await_immediately_v<Awaitable>, int> = 0>
 detail::TryAwaitable<remove_cvref_t<Awaitable>> co_awaitTry(
-    Awaitable&& awaitable) {
+    [[FOLLY_ATTR_CLANG_CORO_AWAIT_ELIDABLE_ARGUMENT]] Awaitable&& awaitable) {
   return detail::TryAwaitable<remove_cvref_t<Awaitable>>{
       static_cast<Awaitable&&>(awaitable)};
 }
 template <
     typename Awaitable,
     std::enable_if_t<is_must_await_immediately_v<Awaitable>, int> = 0>
 detail::TryAwaitable<remove_cvref_t<Awaitable>> co_awaitTry(
-    Awaitable awaitable) {
+    [[FOLLY_ATTR_CLANG_CORO_AWAIT_ELIDABLE_ARGUMENT]] Awaitable awaitable) {
   return detail::TryAwaitable<remove_cvref_t<Awaitable>>{
       std::move(awaitable).unsafeMoveMustAwaitImmediately()};
 }