Allow temporary overrides of error stream in NaClBitcodeParser.

include/llvm/Bitcode/NaCl/NaClBitcodeParser.h

 //===- NaClBitcodeParser.h -----------------------------------*- C++ -*-===//
 //     Low-level bitcode driver to parse PNaCl bitcode files.
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
@@ skipping 450 matching lines @@
   // block with the given block ID, and then call's method
   // ParseThisBlock() to parse the corresponding block. Note:
   // Each derived class should define it's own version of this
   // method, following the pattern below.
   virtual bool ParseBlock(unsigned BlockID) {
     // Default implementation just builds a parser that does nothing.
     NaClBitcodeParser Parser(BlockID, this);
     return Parser.ParseThisBlock();
   }
 
-  // Changes the error stream to print errors to Stream.  Warning:
-  // Stream must exist till the next call to setErrStream.  Otherwise
-  // it must exist for the entire lifetime of the bitcode parser.
-  // Note: This err stream should be set immediately after
-  // construction.  Assigning after nested bitcode parsers have been
-  // built will not work.
-  void setErrStream(raw_ostream &Stream) {
+  // Changes the stream to print errors to, and returns the old error stream.
+  // There are two use cases:
+  // 1) To change (from the default errs()) inside the constructor of the
+  //    derived class. In this context, it will be used for all error
+  //    messages for the derived class.
+  // 2) Temporarily modify it for a single error message. In this context,
+  //    the method Error() is overridden in the derived class, and
+  //    calls this method twice. Once before calling base method Error(),
+  //    and followed by a second call to restore the default error stream.
+  raw_ostream &setErrStream(raw_ostream &Stream) {
+    raw_ostream &OldErrStream = *ErrStream;
     ErrStream = &Stream;
+    return OldErrStream;
   }
 
   // Called when error occurs. Message is the error to report. Always
   // returns true (the error return value of Parse).
   virtual bool Error(const std::string &Message) {
     *ErrStream << "Error: " << Message << "\n";
     return true;
   }
 
   // Returns the number of bits in this block, including nested blocks.
@@ skipping 108 matching lines @@
 
 
   void operator=(const NaClBitcodeParser &Parser) LLVM_DELETED_FUNCTION;
   NaClBitcodeParser(const NaClBitcodeParser &Parser) LLVM_DELETED_FUNCTION;
 
 };
 
 }  // namespace llvm
 
 #endif