N3189 Observers for the three handler functions

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@120712 91177308-0d34-0410-b5e6-96231b3b80d8

include/future

@@ -2297,6 +2297,8 @@ future<_R&>::share()
     return shared_future<_R&>(_STD::move(*this));
 }
 
+#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
+
 inline _LIBCPP_INLINE_VISIBILITY
 shared_future<void>
 future<void>::share()
@@ -2304,6 +2306,8 @@ future<void>::share()
     return shared_future<void>(_STD::move(*this));
 }
 
+#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_FUTURE