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

[Anton Korobeynikov via cfe-commits]

asl wrote:

> Out of curiousity, why don't we have the problem in the normal return continuation ABI?

The problem happens when the value is directly used in `coro.end` intrinsic. For example, when we're forwarding coroutine argument as a result. Or, when the value itself is computed before the suspend. Everything else is correctly handled by the present code due to BB split (the corresponding instructions appear in `Cleanup` block for example and correctly spilled).

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