Add @DiscriminatorMap validation to orm:validate-schema

Assuming that all child classes of an @InheritanceType("SINGLE_TABLE") and @InheritanceType("JOINED") table should be defined in the topmost parent's @DiscriminatorMap, it would be very helpful to add this validation to the CLI orm:valdiate-schema.

At present, if a child table is defined in the topmost table @DiscriminatorMap on a SINGLE_TABLE inheritance, then orm:validate-schema states that the mapping files are correct but then a Doctrine\DBAL\Schema\SchemaException is thrown. The same exception is also thrown for other schema-tool commands (such as 'create').

Haven't tested what happens on a JOINED table..

/**
 * @Entity @InheritanceType("SINGLE_TABLE")
 * @DiscriminatorColumn(name="discr", type="string")
 * @DiscriminatorMap({"employee" = "Employee"})   // note, the second child table was accidentally omitted here
 */
class Person
{
    // ...
}

/**
 * @Entity
 */
class Employee extends Person
{
    // ...
}

/**
 * @Entity
 */
class Customer extends Person
{
    // ....
}

Schema tool commands (create, validate-schema) then throw:

  [Doctrine\DBAL\Schema\SchemaException]
  The table with name 'person' already exists.

... and orm:validate-schema actually says that the mapping is correct (which it obviously isn't) before throwing the exception:

[Mapping]  OK - The mapping files are correct.

  [Doctrine\DBAL\Schema\SchemaException]
  The table with name 'person' already exists.

Not critical since the mapping is obviously wrong here, but having this extra check in the validator could save a lot of debugging time. A duplicate table name error is confusing in this situation, and it seems wrong that the validate-schema command presently reports that the mapping files are correct when in fact the inheritance is broken.