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

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

unintentional add of FunctionAttrs.cpp change?

> we should combine this PR with the PR that adds support for this in LLVM, or else it's weird if we're documenting something that LLVM doesn't support yet

specifically just the attribute support (e.g. bitcode) not any of the inference or usage of it

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