Add GTEST_ATTRIBUTE_UNUSED_ to REGISTER_TYPED_TEST_CASE_P

If REGISTER_TYPED_TEST_CASE_P is included in a header file, but
the .cc file does not declare INSTANTIATE_TYPED_TEST_CASE_P, an
unused-variable warning may be raised by the compiler.

googletest/include/gtest/gtest-typed-test.h

@@ -241,7 +241,8 @@ INSTANTIATE_TYPED_TEST_CASE_P(My, FooTest, MyTypes);
   namespace GTEST_CASE_NAMESPACE_(CaseName) { \
   typedef ::testing::internal::Templates<__VA_ARGS__>::type gtest_AllTests_; \
   } \
-  static const char* const GTEST_REGISTERED_TEST_NAMES_(CaseName) = \
+  static const char* const GTEST_REGISTERED_TEST_NAMES_(CaseName) \
+      GTEST_ATTRIBUTE_UNUSED = \
           GTEST_TYPED_TEST_CASE_P_STATE_(CaseName).VerifyRegisteredTestNames(\
               __FILE__, __LINE__, #__VA_ARGS__)