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

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.

Differential Revision: https://reviews.llvm.org/D97540

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

@@ -22,6 +22,7 @@
 int main(int, char**)
 {
     std::condition_variable cv;
+    static_cast<void>(cv);
 
   return 0;
 }