[llvm] [NFC][Coroutines] Remove integer indexing in several CoroSplit loops (PR #115954)

[Yuxuan Chen via llvm-commits]

yuxuanchen1997 wrote:

For `llvm::enumerate` I think you can also have structured binding instead of `CS.value()` or `CS.index()` calls. 
```
for (const auto &[Index, Value] : llvm::enumerate(Vec)) { ... }
``` 

https://github.com/llvm/llvm-project/pull/115954