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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158609 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/clang/Frontend/ASTUnit.h b/include/clang/Frontend/ASTUnit.h
index 041eabb..c22a18b 100644
--- a/include/clang/Frontend/ASTUnit.h
+++ b/include/clang/Frontend/ASTUnit.h
@@ -396,7 +396,9 @@
   /// 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();