[llvm] [LangRef] Mention allocation elision (PR #177592)

[Ralf Jung via llvm-commits]

https://github.com/RalfJung approved this pull request.

As mentioned in a comment already:

Seems fine to document something based on transformations for now (a strict improvement over the status quo), and open an issue to track that we don't really know what this means operationally and whether it is formally consistent with everything else.

https://github.com/llvm/llvm-project/pull/177592