Created Issue: Code First: TPC with two abstract base classes fails [981]

The following code fails to build a model because there are two abstract base classes before the first concrete class. Removing BillingDetailBase causes the model to be successfully built.

```
public class MyContext : DbContext
{
public DbSet<User> Users { get; set; }

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<BankAccount>().Map(m => { m.ToTable("BankAccounts"); m.MapInheritedProperties(); });
modelBuilder.Entity<CreditCard>().Map(m => { m.ToTable("CreditCards"); m.MapInheritedProperties(); });
}
}

public abstract class BillingDetailBase
{
public Int32 Id { get; set; }
}

public abstract class BillingDetail : BillingDetailBase
{
public string Owner { get; set; }
}

public class User
{
public int UserId { get; set; }
public virtual BillingDetailBase BillingDetail { get; set; }
}

public class BankAccount : BillingDetail
{
public string BankName { get; set; }
public string Swift { get; set; }
}

public class CreditCard : BillingDetail
{
public int CardType { get; set; }
public string ExpiryMonth { get; set; }
public string ExpiryYear { get; set; }
}
```

The error that gets generated is:

```
System.Data.Entity.ModelConfiguration.ModelValidationException: One or more validation errors were detected during model generation:

\tBillingDetailBase: : The referenced EntitySet 'BillingDetailBase' for End 'BillingDetailBase' could not be found in the containing EntityContainer.

at System.Data.Entity.Core.Metadata.Edm.EdmModel.Validate()
at System.Data.Entity.DbModelBuilder.Build(DbProviderManifest providerManifest, DbProviderInfo providerInfo)
at System.Data.Entity.DbModelBuilder.Build(DbConnection providerConnection)
at System.Data.Entity.Internal.LazyInternalContext.CreateModel(LazyInternalContext internalContext)
at System.Data.Entity.Internal.RetryLazy`2.GetValue(TInput input)
at System.Data.Entity.Internal.LazyInternalContext.InitializeContext()
at System.Data.Entity.Internal.LazyInternalContext.get_CodeFirstModel()
at System.Data.Entity.Infrastructure.EdmxWriter.WriteEdmx(DbContext context, XmlWriter writer)
--- End of inner exception stack trace ---
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at System.RuntimeType.InvokeMember(String name, BindingFlags bindingFlags, Binder binder, Object target, Object[] providedArgs, ParameterModifier[] modifiers, CultureInfo culture, String[] namedParams)
at Microsoft.DbContextPackage.Handlers.ViewContextHandler.ViewContext(MenuCommand menuCommand, Object context, Type systemContextType)
```