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

[Jakub Kuderski via llvm-commits]

kuhar wrote:

I don't think we would be gaining much be removing `llvm::identity` in favor of `std::identity` + workarounds for lack of the argument type

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