[llvm] Add the 'initializes' attribute langref and support (PR #84803)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

I think @nikic's concerns have been addressed and there are no major objections to this patch going in. We can address any post-commit feedback with followup PRs, but I'd like to get this in so we can start reviewing the DSE + inference PRs.

https://github.com/llvm/llvm-project/pull/84803