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

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

Thanks for the review!  I take your point.  Given that `IndexedMap.h` is the only user of `llvm::identity`, would you be open to me moving `llvm::identity` to `IndexedMap.h`?  Maybe rename it to `IdentityIndex` wrapped in `namespace detail`?

Otherwise, I am just happy to update comments in `ADT/identity.h` to clarify that it's not really the same thing as `std::identity`.


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