[llvm] [DebugInfo] Make DIArgList inherit from Metadata and always unique (PR #72147)

[Orlando Cazalet-Hyams via llvm-commits]

OCHyams wrote:

LGTM, sounds like a significant win. I would feel more comfortable if another pair of eyes could look over the changes too though.

https://github.com/llvm/llvm-project/pull/72147