[ADT] Make `llvm::identity` a transparent function object

[jloser]

Summary

llvm::identity is similar to std::identity from C++20, but one surprising
thing is that llvm::identity is not a transparent function object. Add the
is_transparent type alias to denote it can be used as a transparent function
object.

[dblaikie]

Sounds good

llvm/include/llvm/ADT/identity.h

 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_ADT_IDENTITY_H
 #define LLVM_ADT_IDENTITY_H
 
 
 namespace llvm {
 
+// Similar to `std::identity` from C++20.
 template <class Ty> struct identity {
+  using is_transparent = void;
   using argument_type = Ty;
 
   Ty &operator()(Ty &self) const {
     return self;
   }
   const Ty &operator()(const Ty &self) const {
     return self;
   }
 };
 
 } // end namespace llvm
 
 #endif // LLVM_ADT_IDENTITY_H