[llvm] [LLVM][Coroutines] Perform HALO on "coro_must_elide" coroutines (PR #98974)

[Chuanqi Xu via llvm-commits]

ChuanqiXu9 wrote:

Some quick feedbacks:

- Since this patch do at least 2 things: create a new ramp function in CoroSplit and introduce a new pass. I think it will be better to split these 2 pases.
- I feel better to do this with attribute instead of metadata. If I rememebr correctly, we don't have a pretty good interface to remove metadata. And I feel it is somewhat unneeded to use metadata here. To pass simple information, I think the attribute is enough and more neat.

https://github.com/llvm/llvm-project/pull/98974