[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 will do a follow-up patch to introduce the new libc++ build to the buildbot.

[rmaprath]

Patch updated to add XFAILS for all those tests that are failing on the new (-fno-exceptions) library variant. Follow-up patches will progressively un-XFAIL these tests (i.e. adapt them to cope with the new library variant).

[jroelofs]

LGTM

[rmaprath]

Thanks Jon!

Committed as r252598. The current buildbots are happy (except for the ARM one, which seem to have gone away).

I will update D14293 to include the new runs.

Cheers,

• Asiri

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();
     }
 }