[CoroEarly] Hide promise alloca for later passes (#139243)
Currently coroutine promises are modeled as allocas. This is problematic
because other middle-end passes will assume promise dead after coroutine
suspend, leading to misoptimizations.
I propose the front ends remain free to emit and use allocas to model
coro promise. At CoroEarly, we will replace all uses of promise alloca
with `coro.promise`. Non coroutine passes should only access promise
through `coro.promise`. Then at CoroSplit, we will lower `coro.promise`
back to promise alloca again. So that it will be correctly collected
into coro frame. Note that we do not have to bother maintainers of other
middle-end passes.
Fix #105595
