Support stored procedure mappings that are agnostic of inheritance mapping strategy

[ajcvickers]

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.