[C++] Use 'nullptr'. Transforms edition.

llvm-svn: 207196
diff --git a/llvm/lib/Transforms/Instrumentation/DebugIR.h b/llvm/lib/Transforms/Instrumentation/DebugIR.h
index 3f57da5..02831ed 100644
--- a/llvm/lib/Transforms/Instrumentation/DebugIR.h
+++ b/llvm/lib/Transforms/Instrumentation/DebugIR.h
@@ -90,7 +90,7 @@
   /// Write M to disk, optionally passing in an fd to an open file which is
   /// closed by this function after writing. If no fd is specified, a new file
   /// is opened, written, and closed.
-  void writeDebugBitcode(const llvm::Module *M, int *fd = 0);
+  void writeDebugBitcode(const llvm::Module *M, int *fd = nullptr);
 };
 
 } // llvm namespace