[libc++] Remove race condition in std::async

Summary:
The state associated to the future was set in one thread (with synchronization)
but read in another thread without synchronization, which led to a data race.

https://bugs.llvm.org/show_bug.cgi?id=38181
rdar://problem/42548261

Reviewers: mclow.lists, EricWF

Subscribers: christof, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D51170

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@340608 91177308-0d34-0410-b5e6-96231b3b80d8

src/future.cpp

@@ -179,10 +179,7 @@ __assoc_sub_state::__execute()
 future<void>::future(__assoc_sub_state* __state)
     : __state_(__state)
 {
-    if (__state_->__has_future_attached())
-        __throw_future_error(future_errc::future_already_retrieved);
-    __state_->__add_shared();
-    __state_->__set_future_attached();
+    __state_->__attach_future();
 }
 
 future<void>::~future()