Fix fields of transient classes being considered duplicate with reportFieldsWhereDeclared

[HypeMC]

When I enabled the reportFieldsWhereDeclared option introduced in #10455, I encountered a problem when using transient classes. For example, the following code produces an error:

abstract class AbstractEntity
{
    #[ORM\Column]
    #[ORM\Id]
    #[ORM\GeneratedValue]
    protected int $id;
}

#[ORM\Entity]
#[ORM\InheritanceType('SINGLE_TABLE')]
#[ORM\DiscriminatorMap(['cat' => Cat::class])]
#[ORM\DiscriminatorColumn(name: 'type')]
abstract class Animal extends AbstractEntity
{
    #[ORM\Column]
    private string $field;
}

#[ORM\Entity]
class Cat extends Animal
{
}

Doctrine\ORM\Mapping\MappingException: Duplicate definition of column 'id' on entity 'Cat' 
in a field or discriminator column mapping.

To my understanding, this shouldn't happen since AbstractEntity is a transient class. Perhaps @mpdude, as the original author, could shed some light on this.

[HypeMC]

@greg0ire @mpdude Sorry to bother you, but could you possibly take a look at this PR? I'm fairly certain this is a bug.

[greg0ire]

Thanks @HypeMC !