[llvm] [IR] LangRef: document behavior of double-lifetime.start (PR #184296)

[Ralf Jung via llvm-commits]

RalfJung wrote:

> This reverts LangRef back to the previous version

Did the LangRef once say what the semantics of this are and then removed it? That seems odd.

Currently behavior is just entirely undocumented, which I would say is different from it being UB. Many things are not documented, often that just means there was an oversight or a good answer is not known.

I don't have a preference for any of the possible options here, I'd just like the docs to be clear and to match what optimizations actually do. Are there currently optimizations that constrain us one way or the other?

That said, @Amanieu has some plans for Rust that wouldn't work if this is UB.

https://github.com/llvm/llvm-project/pull/184296