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

[Arthur Eubanks via llvm-commits]

https://github.com/aeubanks commented:

needs a test in `llvm/test/Bitcode/attributes.ll`, plus some verifier and assembler tests

see tests added in https://github.com/llvm/llvm-project/pull/84617

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