Add deserialization support for section attribute (#71588)

While working on #71425, I hit an assert indicating the `@_section` attribute was not 
handled in `DeclDeserializer::deserializeDeclCommon`. This change adds the missing 
deserialization for the `@_section` attribute.

lib/Serialization/Deserialization.cpp

@@ -6108,6 +6108,14 @@ llvm::Error DeclDeserializer::deserializeDeclCommon() {
         break;
       }
 
+      case decls_block::Section_DECL_ATTR: {
+        bool isImplicit;
+        serialization::decls_block::SectionDeclAttrLayout::readRecord(
+            scratch, isImplicit);
+        Attr = new (ctx) SectionAttr(blobData, isImplicit);
+        break;
+      }
+
       case decls_block::RawLayout_DECL_ATTR: {
         bool isImplicit;
         TypeID typeID;