[clang] Extend `retcon.once` coroutines lowering to optionally produce a normal result (PR #66333)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Got it. Thanks. Then I am wondering how about the `retcon` ABI? I am confused since the doc said the last continuation of `retcon` ABI will return a nullptr to indicate it has finished. Don't we have the same problem? Or we can't solve the problem in `retcon` ABI? Or we just don't want to solve that in the current PR?

https://github.com/llvm/llvm-project/pull/66333