[codeview] Make obj2yaml/yaml2obj support .debug$S/T sections.

This allows us to use yaml2obj and obj2yaml to round-trip CodeView
symbol and type information without having to manually specify the bytes
of the section. This makes for much easier to maintain tests. See the
tests under lld/COFF in this patch for example. Before they just said
SectionData: <blob> whereas now we can use meaningful record
descriptions. Note that it still supports the SectionData yaml field,
which could be useful for initializing a section to invalid bytes for
testing, for example.

Differential Revision: https://reviews.llvm.org/D34127

llvm-svn: 305366

llvm/lib/DebugInfo/CodeView/DebugStringTableSubsection.cpp

@@ -58,6 +58,10 @@ Error DebugStringTableSubsection::commit(BinaryStreamWriter &Writer) const {
   uint32_t Begin = Writer.getOffset();
   uint32_t End = Begin + StringSize;
 
+  // Write a null string at the beginning.
+  if (auto EC = Writer.writeCString(StringRef()))
+    return EC;
+
   for (auto &Pair : Strings) {
     StringRef S = Pair.getKey();
     uint32_t Offset = Begin + Pair.getValue();
@@ -68,6 +72,7 @@ Error DebugStringTableSubsection::commit(BinaryStreamWriter &Writer) const {
   }
 
   Writer.setOffset(End);
+  assert((End - Begin) == StringSize);
   return Error::success();
 }