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

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

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

@kuhar Well, C++ is moving away from `argument_type`.  How about removing the dependence on `argument_type` completely?  Please take a look at the latest iteration.  Thanks!


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