public class GroupBy
{
    public Map<Id, List<SObject>> ids(SObjectField field, List<SObject> records)
    {
        Map<Id, List<SObject>> grouped = new Map<Id, List<SObject>>();
        for (SObject record : records)
        {
            Id parentId = (Id)record.get(field);
            if (!grouped.containsKey(parentId))
                grouped.put(parentId, new List<SObject>());
            grouped.get(parentId).add(record);
        }
        return grouped;
    }
}

List<ThatObject__c> those; // = ...;
Map<Id, List<ThisObject__c>> byParent = GroupBy.ids(ThatObject__c.Parent__c, those);

static List<SObject> genericize(List<SObject> input)
{
    List<SObject> output = new List<SObject>();
    for (SObject record : input) output.add(record);
    return output;
}
List<ThatObject__c> records = genericize([SELECT Id FROM ThisObject__c]);