[llvm] [Coroutines] Model retcon returns explicitly and upgrade legacy IR (PR #227232)

[Weibo He via llvm-commits]

https://github.com/NewSigma commented:

I would like to update the coroutine tests to use the new representation rather than relying on AutoUpgrade.

Specific to the typed-allocator extension, I think we can leave compatibility to https://github.com/llvm/llvm-project/pull/211099.

https://github.com/llvm/llvm-project/pull/227232