Support stored procedure mappings that are agnostic of inheritance mapping strategy

EF6 supports stored procedures with the same shape regardless of the mapping strategy (TPH, TPC, TPT) used for the underlying tables. That is, a single stored procedure for every concrete type that never takes a discriminator--essentially the same shape that EF Core uses for TPC mappings. It is then up to the stored proc to split the data out to multiple tables or set a discriminator as needed.