[llvm] [Support] Add missing LLVM_ABI annotations in Atomic.h (PR #152768)

[Andrew Rogers via llvm-commits]

andrurogerz wrote:

> As for why this had been missed; I had the understanding that the current annotations were made with some automated tool - shouldn't that have caught these declarations?

Correct, the annotations are automatically added by the [IDS tool](https://github.com/compnerd/ids), but it is run on each header individually and this is one was missed. There will likely be more that pop-up over the coming weeks.

https://github.com/llvm/llvm-project/pull/152768