[libcxx] [test] Fix a test error with condvars with trivial destruction

[mstorsjo]

Summary

If the destructor is trivial (_LIBCPP_HAS_TRIVIAL_CONDVAR_DESTRUCTION, the constructor always is), the compiler warns about the std::condition_variable being unused.

Add a cast to void to silence the warning about the object being unused.

libcxx/test/std/thread/thread.condition/thread.condition.condvar/default.pass.cpp

 #include <condition_variable>
 #include <cassert>
 
 #include "test_macros.h"
 
 int main(int, char**)
 {
     std::condition_variable cv;
+    static_cast<void>(cv);
 
   return 0;
 }