[llvm] [LangRef] Update initializes definition (PR #134370)

[Nikita Popov via llvm-commits]

nikic wrote:

Yes, the wording here is for our current semantics where uninitialized memory is undef and memory (de facto if not de jure) cannot contain poison. If we make uninitialized memory in the future, we'd replace undef with poison in this wording as well.

https://github.com/llvm/llvm-project/pull/134370