[pdb] Round trip the PDB stream between YAML and binary PDB.

This gets writing of the PDB stream working.

llvm-svn: 274647
diff --git a/llvm/lib/DebugInfo/PDB/Raw/ModStream.cpp b/llvm/lib/DebugInfo/PDB/Raw/ModStream.cpp
index e1c5e20..3415fcd 100644
--- a/llvm/lib/DebugInfo/PDB/Raw/ModStream.cpp
+++ b/llvm/lib/DebugInfo/PDB/Raw/ModStream.cpp
@@ -78,3 +78,5 @@
 ModStream::lines(bool *HadError) const {
   return llvm::make_range(LineInfo.begin(HadError), LineInfo.end());
 }
+
+Error ModStream::commit() { return Error::success(); }