[llvm] [Utils] Extract ProcessSubprogramAttachment from CloneFunctionInto (PR #114537)

[Felipe de Azevedo Piovezan via llvm-commits]

felipepiovezan wrote:

```
Test Plan:
ninja check-llvm-unit
```

Could you run the `check-llvm`? I don't think the unit tests will run the LIT tests as well, which impact the coro-splitter

https://github.com/llvm/llvm-project/pull/114537