threading_support: delete the critical section

Although the CriticalSection itself doesnt need to be destroyed, there
may be debug data associated with it.  Plug a possible small leak.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291536 91177308-0d34-0410-b5e6-96231b3b80d8

include/__threading_support

@@ -385,7 +385,7 @@ int __libcpp_recursive_mutex_unlock(__libcpp_recursive_mutex_t *__m)
 
 int __libcpp_recursive_mutex_destroy(__libcpp_recursive_mutex_t *__m)
 {
-  static_cast<void>(__m);
+  DeleteCriticalSection(__m);
   return 0;
 }