PR libstdc++/54297
* include/std/future (~_Async_state_impl): Join thread before
derived class members are destroyed.
(~_Async_state_common): Only define non-trivial destructor when
included from future.cc for ABI compatibility reasons.
* src/c++11/future.cc (_GLIBCXX_ABI_COMPAT_ASYNC): Define.
* testsuite/30_threads/async/54297.cc: New.
From-SVN: r190672