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

[Stephen Tozer via llvm-commits]

SLTozer wrote:

Update, doesn't look like anything serious - I can do some serious digging to verify whether the additional dbg.value produced (which is a rare case) is correct or not, but it is not horribly wrong, and if there is anything incorrect about its value or placement it is almost certainly exposed by this patch, rather than caused by it - does that seem good enough to merge this?

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