[llvm] [LangRef] State that the memory model is an axiomatic one (PR #208710)

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

### Merge activity

* **Jul 16, 7:57 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.com/github/pr/llvm/llvm-project/208710).


https://github.com/llvm/llvm-project/pull/208710