Enable building folly/Expected.h with coroutines on and exceptions off

Summary:
`folly::expected_detail::Promise<Value, Error>` won't build with exceptions
turned off (e.g. by passing `-fno-exceptions` on Clang's command line) because
its `unhandled_exception()` says `throw;`.  This diff replaces the `throw;`
with a call to `folly::rethrow_current_exception();`.

Reviewed By: yfeldblum

Differential Revision: D21682605

fbshipit-source-id: 294752590d54b0671395982ccdeb0386a742c01e

folly/Expected.h

@@ -1455,7 +1455,7 @@ struct Promise {
   void unhandled_exception() {
     // Technically, throwing from unhandled_exception is underspecified:
     // https://github.com/GorNishanov/CoroutineWording/issues/17
-    throw;
+    rethrow_current_exception();
   }
 };