[pdb] Refactor library to more clearly separate reading/writing

Reviewed By: amccarth, ruiu
Differential Revision: https://reviews.llvm.org/D22693

llvm-svn: 277019
diff --git a/llvm/lib/DebugInfo/PDB/Raw/ModStream.cpp b/llvm/lib/DebugInfo/PDB/Raw/ModStream.cpp
index 65d59a5..25bb9d1 100644
--- a/llvm/lib/DebugInfo/PDB/Raw/ModStream.cpp
+++ b/llvm/lib/DebugInfo/PDB/Raw/ModStream.cpp
@@ -36,7 +36,7 @@
     return llvm::make_error<RawError>(raw_error_code::corrupt_file,
                                       "Module has both C11 and C13 line info");
 
-  StreamRef S;
+  ReadableStreamRef S;
 
   uint32_t SymbolSubstreamSig = 0;
   if (auto EC = Reader.readInteger(SymbolSubstreamSig))