Update Atomics.rst

Summary:
I changed various bits of the compilation of atomics recently, and forgot
updating the documentation. This patch just brings it up to date.

Test Plan: no change to the code

Reviewers: jfb

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5590
Driver: Use pointee_iterator rather than iterating over unique_ptrs

There's probably never a good reason to iterate over unique_ptrs. This
lets us use range-for and say Job.foo instead of (*it)->foo in a few
places.