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

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

I'm not sure what specification we should use in Alive2 though. Allocations functions can have side-effects and change errno. To allow deletion, we need to model all side-effects as being non-deterministic. @RalfJung does that sound right?

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