[ADT] Move llvm::identity into IndexedMap (NFC) (llvm#164568)

This patch moves llvm::identity to IndexedMap for two reasons:

- llvm::identity is used only IndexedMap.
- llvm::identity is not suitable for general use as it is not quite
  the same as std::identity despite the comments in identity.h.

Also, this patch renames the class to IdentityIndex and places it in
the "detail" namespace.

Author: kazutakahirata

llvm/include/llvm/ADT/IndexedMap.h

@@ -22,12 +22,21 @@
 
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallVector.h"
-#include "llvm/ADT/identity.h"
 #include <cassert>
 
 namespace llvm {
 
-template <typename T, typename ToIndexT = identity<unsigned>> class IndexedMap {
+namespace detail {
+template <class Ty> struct IdentityIndex {
+  using argument_type = Ty;
+
+  Ty &operator()(Ty &self) const { return self; }
+  const Ty &operator()(const Ty &self) const { return self; }
+};
+} // namespace detail
+
+template <typename T, typename ToIndexT = detail::IdentityIndex<unsigned>>
+class IndexedMap {
   using IndexT = typename ToIndexT::argument_type;
   // Prefer SmallVector with zero inline storage over std::vector. IndexedMaps
   // can grow very large and SmallVector grows more efficiently as long as T