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

[Anton Korobeynikov via cfe-commits]

asl 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?

Ah, ok. The current PR is only for `retcon.once` coroutines. I do not have a good use-case for `retcon` ones and they are lowered differently enough, so I decided not to touch them. If / when there will be a viable usecase, I will try to generalize the present implementation to generic `retcon` coroutines.

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