[clang] [CIR] Add missing coroutine plumbing (PR #222472)

[Vlad Sytchenko via cfe-commits]

vsytch wrote:

> @Andres-Salamanca has been working on the coroutine support for quite a while (blocked mostly by my slowness in review), and I'm concerned that parts of this will diverge from what he has planned following #213191).

Thank you for letting me know! I'll close this chain then.

On a side note, is there a CIR CORO TODO list? I'm using CIR for unrelated reasons, but coroutine support is crucial for my case, so I'd be willing to help as much as I can.

https://github.com/llvm/llvm-project/pull/222472