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

[Haopeng Liu via llvm-commits]

haopliu wrote:

Good point. Changed the attr impl to use normal Alloc w/ a vector and explicitly call each attr's dtor in LLVMContext dtor. Please take another look. Thanks!

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