Apply clang-tidy modernize-use-nullptr to googletest.

Now that googletest has moved to C++11, it should no longer
use NULL or 0 for the null pointer. This patch converts all
such usages to nullptr using clang-tidy.

This prevents LLVM from issuing -Wzero-as-null-pointer-constant
warnings.

PiperOrigin-RevId: 215814400

googletest/test/gtest_stress_test.cc

@@ -163,7 +163,7 @@ void FailingThread(bool is_fatal) {
 }
 
 void GenerateFatalFailureInAnotherThread(bool is_fatal) {
-  ThreadWithParam<bool> thread(&FailingThread, is_fatal, NULL);
+  ThreadWithParam<bool> thread(&FailingThread, is_fatal, nullptr);
   thread.Join();
 }