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

[Yuxuan Chen via llvm-commits]

yuxuanchen1997 wrote:

> * 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.

I also think Attributes are neater if they work. Instructions don't have attributes but CallBase does. I tried to add Attributes on the CallBase but however the `dump()` doesn't show it somehow. It also gave me the impression that Attributes on `CallBase` is specifically for parameters. 

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