[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);
> > ```
> 
> You didn't implement a virtual method of the base class. If you look further along in the error message it should tell you which ones

@arsenm, Thanks.
Would you take a look? how to use  `AAConvertOutArgumentFunction` It's not changing the sample function in the tests?

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