Restore TfLitePtrUnion.raw_const - removing it was breaking the API.

PiperOrigin-RevId: 273601865
diff --git a/tensorflow/lite/c/c_api_internal.h b/tensorflow/lite/c/c_api_internal.h
index 08b0ee7..7a525ef 100644
--- a/tensorflow/lite/c/c_api_internal.h
+++ b/tensorflow/lite/c/c_api_internal.h
@@ -269,6 +269,7 @@
   // Placeholder for 16b float type. Use uint16* in the pointer union for now.
   TfLiteFloat16* f16;
   char* raw;
+  const char* raw_const;
   uint8_t* uint8;
   bool* b;
   int16_t* i16;
diff --git a/tensorflow/lite/experimental/c/c_api_types.h b/tensorflow/lite/experimental/c/c_api_types.h
index 08b0ee7..7a525ef 100644
--- a/tensorflow/lite/experimental/c/c_api_types.h
+++ b/tensorflow/lite/experimental/c/c_api_types.h
@@ -269,6 +269,7 @@
   // Placeholder for 16b float type. Use uint16* in the pointer union for now.
   TfLiteFloat16* f16;
   char* raw;
+  const char* raw_const;
   uint8_t* uint8;
   bool* b;
   int16_t* i16;