Guard private fields that are unused in Release builds with #ifndef NDEBUG.

llvm-svn: 158609

clang/include/clang/Frontend/ASTUnit.h

@@ -396,7 +396,9 @@ private:
   /// just about any usage.
   /// Becomes a noop in release mode; only useful for debug mode checking.
   class ConcurrencyState {
+#ifndef NDEBUG
     void *Mutex; // a llvm::sys::MutexImpl in debug;
+#endif
 
   public:
     ConcurrencyState();