[C++] Use 'nullptr'.

llvm-svn: 207394
diff --git a/llvm/lib/DebugInfo/DWARFUnit.h b/llvm/lib/DebugInfo/DWARFUnit.h
index 374ecea..471da36 100644
--- a/llvm/lib/DebugInfo/DWARFUnit.h
+++ b/llvm/lib/DebugInfo/DWARFUnit.h
@@ -118,7 +118,7 @@
   const DWARFDebugInfoEntryMinimal *
   getCompileUnitDIE(bool extract_cu_die_only = true) {
     extractDIEsIfNeeded(extract_cu_die_only);
-    return DieArray.empty() ? NULL : &DieArray[0];
+    return DieArray.empty() ? nullptr : &DieArray[0];
   }
 
   const char *getCompilationDir();