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

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

This patch adds XFAILS to all those tests that are currently failing
on the new -fno-exceptions library variant. Follow-up patches will
update the tests (progressively) to cope with the new library variant.

Change-Id: I4b801bd8d8e4fe7193df9e55f39f1f393a8ba81a

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@252598 91177308-0d34-0410-b5e6-96231b3b80d8

src/future.cpp

@@ -221,10 +221,12 @@ promise<void>::~promise()
 {
     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();
     }
 }