Make ASTUnit structure stable with NDEBUG

ASTUnit instances are allocated infrequently so it's fine to keep this field
around in all build configurations.

Assigns null to silence -Wunused-private-field in Release.

llvm-svn: 195419

clang/include/clang/Frontend/ASTUnit.h

@@ -406,9 +406,7 @@ 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();

clang/lib/Frontend/ASTUnit.cpp

@@ -2953,7 +2953,7 @@ void ASTUnit::ConcurrencyState::finish() {
 
 #else // NDEBUG
 
-ASTUnit::ConcurrencyState::ConcurrencyState() {}
+ASTUnit::ConcurrencyState::ConcurrencyState() { Mutex = 0; }
 ASTUnit::ConcurrencyState::~ConcurrencyState() {}
 void ASTUnit::ConcurrencyState::start() {}
 void ASTUnit::ConcurrencyState::finish() {}