[clang][index-while-building][NFC] Comment about implementation detail in FileIndexRecord

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@355061 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Index/FileIndexRecord.cpp

@@ -35,6 +35,7 @@ void FileIndexRecord::addDeclOccurence(SymbolRoleSet Roles, unsigned Offset,
   }
 
   DeclOccurrence NewInfo(Roles, Offset, D, Relations);
+  // We keep Decls in order as we need to access them in this order in all cases.
   auto It = std::upper_bound(Decls.begin(), Decls.end(), NewInfo);
   Decls.insert(It, std::move(NewInfo));
 }