[libcxx] Attempt to fix __throw_future_error in C++03

Summary:
Hi Marshall,

Could you please test this patch and see if you run into the same linker errors we talked about?
I can't reproduce on linux or OS X.

Hopefully you can't find any problems and we can fix the C++03 bot.

Reviewers: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D13337

llvm-svn: 249192