Allow temporary overrides of error stream in NaClBitcodeParser.

include/llvm/Bitcode/NaCl/NaClBitcodeParser.h

Index: include/llvm/Bitcode/NaCl/NaClBitcodeParser.h
diff --git a/include/llvm/Bitcode/NaCl/NaClBitcodeParser.h b/include/llvm/Bitcode/NaCl/NaClBitcodeParser.h
index f9c02f71ad9e09a0b73fad9fe7502a986a641436..97f09ce34e8651b2c94c07451fbd5275a1bfc3de 100644
--- a/include/llvm/Bitcode/NaCl/NaClBitcodeParser.h
+++ b/include/llvm/Bitcode/NaCl/NaClBitcodeParser.h
@@ -468,14 +468,18 @@ public:
     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. There are two use cases:

[Jim Stichnoth, 2015/01/22 17:26:13]

Just to be clear for the short-of-attention-span, add a note in the first
sentence saying it returns the old stream.

[Karl, 2015/01/22 17:42:58]

Done.

+  // 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) {

[Jim Stichnoth, 2015/01/22 17:26:13]

It seems a little odd to mix raw_ostream* and raw_ostream&.  Would it be
reasonable to return raw_ostream& instead?

[Karl, 2015/01/22 17:42:58]

Done.

+    raw_ostream *OldErrStream = ErrStream;
     ErrStream = &Stream;
+    return OldErrStream;
   }
 
   // Called when error occurs. Message is the error to report. Always