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

[Haopeng Liu via llvm-commits]

haopliu wrote:

Addressed Arthur's and Jan's comments, and added unit tests for BitcodeReader/Verifier/Assembler!
Please take another look :-D
Gentle ping other reviewers. Thanks!

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