[llvm] Add debuginfo C support for a SetType, Subrangetype, dynamic array type and replace arrays (PR #135607)

[Orlando Cazalet-Hyams via llvm-commits]

OCHyams wrote:

I was just having a look and there's a third option (first on this list) - 

* `unwrap()` can take null, because it `reinterpret_cast`s
* `unwrap<T>()` casts using `casat`, so can't take nulls
* `unwrapDI<T>()` checks for null explicitly 

It's not particularly clear at all, I was also a bit confused there trying to understand why `unwrap(BitStride)` was ok with null.

> Takes me ages to build the system with my old laptop, literally days.

I've left a comment in line. I appreciate this has been a bit of a protracted review, sorry about that. If making those final changes is going to be difficult with your hardware I might be able to take it over, if that's helpful (can't promise when though).

https://github.com/llvm/llvm-project/pull/135607