[llvm] [PassInstrumentation] Replace llvm::Any with a tagged IRUnitRef. (NFC) (PR #215341)

[Alexis Engelke via llvm-commits]

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

LGTM, thanks. (NB: this removes our only use of llvm::Any in llvm/, the few remaining uses are in clang/.)

https://github.com/llvm/llvm-project/pull/215341