After a long break to wait for the atomic spec to settle, this completes the...

After a long break to wait for the atomic spec to settle, this completes the library part of <atomic>.  It currently won't even parse as it depends on the existence of the intrinsics specified at http://libcxx.llvm.org/atomic_design_a.html.  Everything has been tested using fake intrinsics which have now been removed.  As the intrinsics come online, the ATOMIC_* macros will need to be adjusted to reflect which operations are lock-free.  These macros will probably need to be #ifdef'd for each supported platform.

llvm-svn: 121267