[llvm] [CoroEarly] Add presplitcoroutine when frontend misses it (PR #169866)

[Chuanqi Xu via llvm-commits]

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ChuanqiXu9 wrote:

Can we set it directly without the if check? And also I feel better to remove the statements in clang which set the attribute with this patch.

https://github.com/llvm/llvm-project/pull/169866