[llvm] [Attributor] Pack out arguments into a struct (PR #119267)

[via llvm-commits]

elhewaty wrote:

@arsenm @shiltian @vidsinghal @jdoerfert  @jhuber6 
I am sorry but this is my first time with the attributor. I keep getting compilation errors like:
```
error: invalid new-expression of abstract class type ‘{anonymous}::AAConvertOutArgumentFunction’
13207 |     AA = new (A.Allocator) CLASS##SUFFIX(IRP, A);
```

But I need you to share your opinions so far, I will write tests to cover all possible cases, but at least I need this to work with the simple test function written before.

What did I mess up while adding the attributor?
I think I need to change the whole thing, but this is my best for now!


https://github.com/llvm/llvm-project/pull/119267