[llvm] [ADT] Modernize IndexedMap to use llvm::identity_cxx20 (NFC) (PR #164568)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar commented:

Shouldn't we retain `llvm::identity` for cases like this? It seems much cleaner to me.

https://github.com/llvm/llvm-project/pull/164568