[PATCH] [libcxx] Allow <atomic> to be used in c++03.

[Dan Albert]

Not sure what our policy is on allowing C++11 features in pre-C++11 code. I think I'd prefer that people just be required to upgrade to C++11 rather than allowing them to keep living in the past.

I also don't like that we're not implementing a complete `<atomic>` in C++03 mode. Sounds like another good reason to not support it to me.

I'll defer to mclow on this one. Just my two cents.

http://reviews.llvm.org/D6284