[llvm] [MIR] Replace bespoke DIExpression parser (PR #96827)

[Orlando Cazalet-Hyams via llvm-commits]

https://github.com/OCHyams commented:

I think this LGTM now, ty. It still would be good if someone else could glance at it too, cc @SLTozer who has poked around in the parser more recently.

https://github.com/llvm/llvm-project/pull/96827