[llvm] coro.retcon: Fix return type and buffer poisoning in elided coroutines (PR #208384)

[Chuanqi Xu via llvm-commits]

https://github.com/ChuanqiXu9 commented:

I am not familiar with RetCon ABI. @rjmccall would like to take a look or find someone else maintaining this right now?

https://github.com/llvm/llvm-project/pull/208384