[llvm] [LangRef] Clarify that the pointer after an objet must be valid. (PR #127892)

[Nikita Popov via llvm-commits]

nikic wrote:

> Not sure if there is a better place to document the assumption than in the GEP semantics

It would make sense to add a separate section on allocated objects.

https://github.com/llvm/llvm-project/pull/127892