[rtsan] Add test for pthread_cond_wait segfault (#150776)

Introduce the test from
#146120

For future readers of this PR, if this test causes a segfault please
comment out the line indicated by the comment (or revert this entire
commit).

My plan is to commit this, see if any test runners fail, then submit the
fix in a follow on.

I cannot repro this bug on my machine so I need some confirmation of the
bug being fixed as it is submitted.

Author: cjappl

compiler-rt/test/rtsan/pthread_cond_wait.cpp

@@ -0,0 +1,46 @@
+// RUN: %clangxx -fsanitize=realtime %s -o %t
+// RUN: %run %t 2>&1 | FileCheck %s
+
+// UNSUPPORTED: ios
+
+// Intent: Ensures that pthread_cond_signal does not segfault under rtsan
+// See issue #146120
+
+#include <condition_variable>
+#include <future>
+#include <mutex>
+#include <thread>
+
+#include <iostream>
+
+int main() {
+  std::cout << "Entry to main!" << std::endl;
+  std::mutex mut;
+  std::condition_variable cv;
+  bool go{false};
+
+  const auto fut = std::async(std::launch::async, [&] {
+    std::this_thread::sleep_for(std::chrono::milliseconds(100));
+    {
+      std::unique_lock<std::mutex> lock(mut);
+      go = true;
+    }
+    cv.notify_one();
+  });
+
+  std::unique_lock<std::mutex> lock(mut);
+  // normal wait is fine
+  // cv.wait(lock, [&] { return go; });
+  // but timed wait could segfault
+
+  // NOTE: If this test segfaults on a test runner, please comment
+  //       out this line and submit the patch.
+  //       I will follow up with a fix of the underlying problem,
+  //       but first I need to understand if it fails a test runner
+  cv.wait_for(lock, std::chrono::milliseconds(200), [&] { return go; });
+
+  std::cout << "Exit from main!" << std::endl;
+}
+
+// CHECK: Entry to main!
+// CHECK-NEXT: Exit from main!