[llvm] [RFC][LangRef] Specify that the accessed bytes of concurrent atomics must be either disjoint or the same (PR #204329)

[Ralf Jung via llvm-commits]

RalfJung wrote:

> I don't agree that the current wording for elementwise necessarily allows that. seq_cst accesses in different threads aren't intrinsically ordered either, but the seq_cst order needs to order them.
If this is the intended behavior, it needs to be said more clearly there.

Agreed; I would interpret "unordered" as "as-if in concurrent threads".

But anyway the conclusion in the meeting was that we'll disallow seq_cst for elementwise for now so this question should disappear.

https://github.com/llvm/llvm-project/pull/204329