[ADT] Stop including identity.h (NFC)

The last use of identity was removed by:

  commit 388b8c16
  Author: Benjamin Kramer <benny.kra@googlemail.com>
  Date:   Wed Jan 25 01:38:28 2023 +0100

While I am at it, this patch teaches IndexedMap.h to include
identity.h as it is relying on transitive includes via
llvm/ADT/STLExtras.h.