Install notion of diagnostic handler into PNaCl bitcode readers.

unittests/Bitcode/NaClMungeTest.h

Index: unittests/Bitcode/NaClMungeTest.h
diff --git a/unittests/Bitcode/NaClMungeTest.h b/unittests/Bitcode/NaClMungeTest.h
index 52c7d93ddf3b2816e8db28de6cd58883e081d7bc..8d8b0263d63b8a3ebb0aad55dd42b1883566d6cd 100644
--- a/unittests/Bitcode/NaClMungeTest.h
+++ b/unittests/Bitcode/NaClMungeTest.h
@@ -25,6 +25,10 @@ const uint64_t Terminator = 0x5768798008978675LL;
 
 #define ARRAY_TERM(name) ARRAY(name), Terminator
 
+// Removes error severity prefices, if present, for each line in the message.
+// Returns the stripped result.
+std::string stripErrorPrefix(const std::string &Message);
+
 inline std::string stringify(llvm::NaClMungedBitcode &MungedBitcode) {
   std::string Buffer;
   llvm::raw_string_ostream StrBuf(Buffer);