[llvm] [Coroutines] Implement elision for noop coroutines (PR #174353)

[Weibo He via llvm-commits]

NewSigma wrote:

Thanks for the code review. I agree with you that CoroElide is designed for coroutine allocation. I worked on CoroElide because

1. The approach requires that we see the `coro.noop` intrinsic, which may be exposed by other middle end optimizations.
2. The pattern is not restricted to presplitcoroutine. Consider:

``` C++
void fn() {
  std::noop_coroutine().resume();
}
```

Not sure if there is a realistic use case, though.

I tried to handle await_suspend specifically in CoroSplit, but optimizations do not work well for await_suspend wrappers(*), which is a blocking issue. In this revision, I erase `coro.noop` in CoroCleanup and removed unrelated changes, since CoroCleanup is late in pipeline and works for general functions.

(*) It seems clang forgot to add function attributes to await_suspend wrappers. The inliner refuses to inline callees into await_suspend because their attributes mismatch.

https://github.com/llvm/llvm-project/pull/174353