Replace direct usages of std coroutines with folly::coro in folly::result

Summary: Fixes compile errors when coroutines are under `experimental` in the active compiler.

Reviewed By: praihan

Differential Revision: D79929244

fbshipit-source-id: 8088e31088a85c2714c3300fd6fc3c9f97d3dae9

Author: hchokshi

folly/result/BUCK

@@ -46,6 +46,7 @@ non_fbcode_target(
         "//xplat/folly:expected",
         "//xplat/folly:operation_cancelled",
         "//xplat/folly:portability_gtest_prod",
+        "//xplat/folly/coro:coroutine",
         "//xplat/folly/lang:align",
         "//xplat/folly/lang:rvalue_reference_wrapper",
     ],
@@ -63,6 +64,7 @@ fbcode_target(
         "//folly:exception_wrapper",
         "//folly:expected",
         "//folly:operation_cancelled",
+        "//folly/coro:coroutine",
         "//folly/lang:align",
         "//folly/lang:rvalue_reference_wrapper",
         "//folly/portability:gtest_prod",

folly/result/result.h

@@ -19,6 +19,7 @@
 #include <folly/ExceptionWrapper.h>
 #include <folly/Expected.h>
 #include <folly/OperationCancelled.h>
+#include <folly/coro/Coroutine.h>
 #include <folly/lang/Align.h> // for `hardware_constructive_interference_size`
 #include <folly/lang/RValueReferenceWrapper.h>
 #include <folly/portability/GTestProd.h>
@@ -739,10 +740,10 @@ struct result_promise_base {
   void operator=(result_promise_base&&) = delete;
   ~result_promise_base() = default;
 
-  FOLLY_NODISCARD std::suspend_never initial_suspend() const noexcept {
+  FOLLY_NODISCARD folly::coro::suspend_never initial_suspend() const noexcept {
     return {};
   }
-  FOLLY_NODISCARD std::suspend_never final_suspend() const noexcept {
+  FOLLY_NODISCARD folly::coro::suspend_never final_suspend() const noexcept {
     return {};
   }
   void unhandled_exception() noexcept {
@@ -778,7 +779,7 @@ struct result_promise<T, typename std::enable_if<std::is_void_v<T>>::type>
 };
 
 template <typename T>
-using result_promise_handle = std::coroutine_handle<result_promise<T>>;
+using result_promise_handle = folly::coro::coroutine_handle<result_promise<T>>;
 
 // This is separate to let `result_generator` reuse the awaitables below.
 struct result_await_suspender {