[codeview] Fully initialize DataSym when mapping from YAML

In the object file, the section index and relative offset are typically
zero, so make these YAML fields optional with a default.

It looks like there may be more partially initialized symbol records,
but this should fix the msan bot.

llvm-svn: 305842
diff --git a/llvm/lib/ObjectYAML/CodeViewYAMLSymbols.cpp b/llvm/lib/ObjectYAML/CodeViewYAMLSymbols.cpp
index 2f78676..edabe41 100644
--- a/llvm/lib/ObjectYAML/CodeViewYAMLSymbols.cpp
+++ b/llvm/lib/ObjectYAML/CodeViewYAMLSymbols.cpp
@@ -489,6 +489,8 @@
 template <> void SymbolRecordImpl<DataSym>::map(IO &IO) {
   // TODO: Map linkage name
   IO.mapRequired("Type", Symbol.Type);
+  IO.mapOptional("DataOffset", Symbol.DataOffset, 0U);
+  IO.mapOptional("Segment", Symbol.Segment, uint16_t(0));
   IO.mapRequired("DisplayName", Symbol.Name);
 }