[llvm] [CoroEarly] Hide promise alloca for later passes (PR #139243)

[Chuanqi Xu via llvm-commits]

ChuanqiXu9 wrote:

> > Now how does CoroFrame pass generate the coroutine frame and generate the promise field correctly?
> 
> We lower coro.promise before buildFrameType(), so the CoroFrame pass won't see any difference and should work as it used to be.

My question is, preivously, there were many logics about PromiseAlloca in CoroFrame. Then now, why the code can work properly without touching these logic? And if it does work intentionally, we should cleanup these logics.

https://github.com/llvm/llvm-project/pull/139243