[ADT] Stop including identity.h (NFC)

The last use of identity was removed by:

  commit 388b8c1
  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.

Author: kazutakahirata

llvm/include/llvm/ADT/IndexedMap.h

@@ -22,6 +22,7 @@
 
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/identity.h"
 #include <cassert>
 
 namespace llvm {

llvm/include/llvm/ADT/STLExtras.h

@@ -21,7 +21,6 @@
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/STLForwardCompat.h"
 #include "llvm/ADT/STLFunctionalExtras.h"
-#include "llvm/ADT/identity.h"
 #include "llvm/ADT/iterator.h"
 #include "llvm/ADT/iterator_range.h"
 #include "llvm/Config/abi-breaking.h"