… types of discriminated classes (sandergo90)

This PR was merged into the 4.2 branch.

Discussion
----------

[Serializer] AbstractObjectNormalizer ignores the property types of discriminated classes

| Q             | A
| ------------- | ---
| Branch?       | 4.2
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #27607
| License       | MIT
| Doc PR        |

As discusses in ticket #27607, when using a discriminator map and the symfony serializer, things go wrong when having two classes in the discriminator map having the same property name. When the function ```validateAndDenormalize``` is called, the function ```getTypes``` would be called with the wrong class name.

If you take a look at the ```getTypes``` function below, I'm not even sure if we still need the part of the discriminator at this place because we already passed the class name of the discriminated class.

```
    /**
     * @return Type[]|null
     */
    private function getTypes(string $currentClass, string $attribute)
    {
        if (null === $this->propertyTypeExtractor) {
            return null;
        }

        if (null !== $types = $this->propertyTypeExtractor->getTypes($currentClass, $attribute)) {
            return $types;
        }

        if (null !== $this->classDiscriminatorResolver && null !== $discriminatorMapping = $this->classDiscriminatorResolver->getMappingForClass($currentClass)) {
            if ($discriminatorMapping->getTypeProperty() === $attribute) {
                return [
                    new Type(Type::BUILTIN_TYPE_STRING),
                ];
            }

            foreach ($discriminatorMapping->getTypesMapping() as $mappedClass) {
                if (null !== $types = $this->propertyTypeExtractor->getTypes($mappedClass, $attribute)) {
                    return $types;
                }
            }
        }

        return null;
    }
```

Commits
-------

9fc56c7 [Serializer]: AbstractObjectNormalizer ignores the property types of discriminated classes