[libcxx] Make it possible to build a no-exceptions variant of libcxx.

[rmaprath]

Summary

Fixes a small omission in libcxx that prevents libcxx being built when -DLIBCXX_ENABLE_EXCEPTIONS=0 is used.

I have a separate patch to add a no-exceptions build of libcxx to zorg in D14287 (blocked on the current patch).

[rmaprath]

Abandoning revision as it seems I cannot add subscribers/reviewers to existing phab reviews.

src/future.cpp

 {
     if (__state_)
     {
+#ifndef _LIBCPP_NO_EXCEPTIONS
         if (!__state_->__has_value() && __state_->use_count() > 1)
             __state_->set_exception(make_exception_ptr(
                       future_error(make_error_code(future_errc::broken_promise))
                                                       ));
+#endif // _LIBCPP_NO_EXCEPTIONS
         __state_->__release_shared();
     }
 }