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

[Haopeng Liu via llvm-commits]

haopliu wrote:

Here is the new compile time cost for the entire stack of patches:
(Attr support with CRL + Attr inference + Apply in DSE).

https://llvm-compile-time-tracker.com/compare.php?from=07d7b9c255078edc6f04bd4e68416bdf3e8735ab&to=3d53c17c11aec6d71a35c5a80ef91764b23efb52&stat=instructions%3Au

See the detail in ["haopliu/perf/initialized-attr-CRL"](https://llvm-compile-time-tracker.com/index.php?config=Overview&stat=instructions%3Au&remote=haopliu).

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