Mark RefCountPtr as a nullable type.

This allows uses of the type to be annotated as absl_nonnull, absl_nullable, or absl_nullability_unknown.

It's possible that the clang compiler could be altered to automatically consider classes as nullable if they inherit from a nullable class, but that won't happen any time soon.

PiperOrigin-RevId: 797525969

Author: tensorflower-gardener

tsl/platform/BUILD

@@ -1213,6 +1213,7 @@ cc_library(
     deps = [
         ":mutex",
         ":thread_annotations",
+        "@com_google_absl//absl/base:nullability",
         "@xla//xla/tsl/platform:logging",
     ],
 )

tsl/platform/refcount.h

@@ -20,6 +20,7 @@ limitations under the License.
 #include <map>
 #include <memory>
 
+#include "absl/base/nullability.h"
 #include "xla/tsl/platform/logging.h"
 #include "tsl/platform/mutex.h"
 #include "tsl/platform/thread_annotations.h"
@@ -98,7 +99,8 @@ ABSL_MUST_USE_RESULT RefCountPtr<T> GetNewRef(T* ptr) {
 
 // A unique_ptr that unrefs the owned object on destruction.
 template <typename T>
-class RefCountPtr : public std::unique_ptr<T, RefCountDeleter> {
+class ABSL_NULLABILITY_COMPATIBLE RefCountPtr
+    : public std::unique_ptr<T, RefCountDeleter> {
  public:
   using std::unique_ptr<T, RefCountDeleter>::unique_ptr;
   ABSL_MUST_USE_RESULT RefCountPtr GetNewRef() const {