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

[Jeremy Morse via llvm-commits]

jmorse wrote:

I was a bit skeptical about this ^, after some back and forth with @SLTozer though we reckon it's probably `isIdenticalToWhenDefined` returning something different now when merging blocks in SimplifyCFG, which would result in dbg.values not-being-dropped where they would have been before. With that in mind, LGTM again.

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