[llvm] Reapply "[coro][CoroSplit] Use `llvm.lifetime.end` to compute putting objects on the frame vs the stack (#90265) (PR #91372)

[Nikita Popov via llvm-commits]

nikic wrote:

@alanzhao1 std::variant is inefficient on multiple levels. It is very unlikely to be suitable in cases where the whole point is to have efficient code.

https://github.com/llvm/llvm-project/pull/91372