[llvm] [RemoveDIs][AsmWriter] Add empty-metadata operands to the SlotTracker (PR #85636)

[Stephen Tozer via llvm-commits]

https://github.com/SLTozer approved this pull request.

Think there's a FileCheck variable redefinition error in the test, but otherwise LGTM.

https://github.com/llvm/llvm-project/pull/85636