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
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index a8c5876..cbdfab5 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -2953,7 +2953,7 @@
 
 #else // NDEBUG
 
-ASTUnit::ConcurrencyState::ConcurrencyState() {}
+ASTUnit::ConcurrencyState::ConcurrencyState() { Mutex = 0; }
 ASTUnit::ConcurrencyState::~ConcurrencyState() {}
 void ASTUnit::ConcurrencyState::start() {}
 void ASTUnit::ConcurrencyState::finish() {}