Make gtl/int_type.h types hashable in ABSL containers.

(Note ABSL hash functions are templated, and so there's no cost if you don't use them.)

PiperOrigin-RevId: 294764409
Change-Id: I19bafd4a19eb6b623a48ce5d136dd5bf6c20e56b

tensorflow/core/lib/gtl/int_type.h

@@ -197,6 +197,11 @@ class IntType {
     }
   };
 
+  template <typename H>
+  friend H AbslHashValue(H h, const IntType& i) {
+    return H::combine(std::move(h), i.value());
+  }
+
  public:
   // Default c'tor initializing value_ to 0.
   constexpr IntType() : value_(0) {}