AlterColumnOperation.OldColumn is missing some details

The OldColumn on AlterColumnOperation has null Schema, Table and Name - these are only populated on the main column. I get that these should be identical between the old and new column, but in various code which treats the old column separately (e.g. to look up the table in the relational model), this causes some friction (nothing too serious).

The relevant code is here: https://github.com/dotnet/efcore/blob/main/src/EFCore.Relational/Migrations/Internal/MigrationsModelDiffer.cs#L1002