[llvm] Add 'initialized' attribute langref (PR #84803)

[Haopeng Liu via llvm-commits]

haopliu wrote:

> As a heads up, the inference implementation for this (or at least the available prototype) does not look viable from a compile-time perspective: https://llvm-compile-time-tracker.com/compare.php?from=69b09b43b0a2057918078edb401adab888d1014b&to=0bd68ae2d56783377acc0aa5d7958b47411b8342&stat=instructions:u

Thanks for this heads up! This PR only adds the attribute support. Will further tune the infering performance and post the updated compile time tracker in [the inference PR](https://github.com/llvm/llvm-project/pull/84869).

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