[C++] Use 'nullptr'.

llvm-svn: 207394
diff --git a/llvm/lib/Target/ARM/ARMTargetObjectFile.h b/llvm/lib/Target/ARM/ARMTargetObjectFile.h
index 5f8d612..c926421 100644
--- a/llvm/lib/Target/ARM/ARMTargetObjectFile.h
+++ b/llvm/lib/Target/ARM/ARMTargetObjectFile.h
@@ -23,7 +23,7 @@
 public:
   ARMElfTargetObjectFile() :
     TargetLoweringObjectFileELF(),
-    AttributesSection(NULL)
+    AttributesSection(nullptr)
   {}
 
   void Initialize(MCContext &Ctx, const TargetMachine &TM) override;