[Gardening] Do not store start/end line in SingleRawComment

The start and end lines were only used while constructing the comments,
so move the line tracking into that method instead of storing it in each
comment.

lib/Serialization/SerializeDoc.cpp

@@ -247,7 +247,7 @@ public:
     out << data.Brief;
     writer.write<uint32_t>(data.Raw.Comments.size());
     for (auto C : data.Raw.Comments) {
-      writer.write<uint32_t>(C.StartColumn);
+      writer.write<uint32_t>(C.ColumnIndent);
       writer.write<uint32_t>(C.RawText.size());
       out << C.RawText;
     }