[llvm] Fix non-determinism in debuginfo (PR #68332)

[Paul Kirth via llvm-commits]

ilovepi wrote:

huh, not sure why `check-llvm` was passing for me locally, since it looks like the change w/ `std::pair` is just broken. 

I'm probably going to go back to the DenseMapInfo Specialization, unless this ends up being simple to fix.

https://github.com/llvm/llvm-project/pull/68332