[Serialization] Avoid using BCRecordLayout for decoding swiftdocs

BCRecordLayout currently assumes that the layout described in source
always matches the layout in the bitstream being read. Since we want
swiftdocs to be a forward-compatible format, avoid using it for
deserialization.

(In the future, we may want to augment BCRecordLayout to handle
records with more fields than expected. For now, though, this is a
sufficient change.)

lib/Serialization/ModuleFile.cpp

@@ -955,8 +955,9 @@ public:
 std::unique_ptr<ModuleFile::SerializedDeclCommentTable>
 ModuleFile::readDeclCommentTable(ArrayRef<uint64_t> fields,
                                  StringRef blobData) {
-  uint32_t tableOffset;
-  index_block::DeclListLayout::readRecord(fields, tableOffset);
+  if (fields.empty() || blobData.empty())
+    return nullptr;
+  uint32_t tableOffset = static_cast<uint32_t>(fields.front());
   auto base = reinterpret_cast<const uint8_t *>(blobData.data());
 
   return std::unique_ptr<SerializedDeclCommentTable>(