symfony

This PR was squashed before being merged into the 2.7 branch (closes #21267).

Discussion
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[Form] Fix ChoiceType to ensure submitted data is not nested unnecessarily

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Fixed ChoiceType to protect against some problem caused by treating of array.

Let's say we have the choice-form like:

```php
$form = $factory->create(ChoiceType, null, [
    'choices' => [
        'A',
        'B',
        'C',
    ],
    'expanded' => true,
    'multiple' => true,
]);
```

Then, submit data like this:

```php
$form->submit([
    [], // unnecessality nested
]);
```

(Yes, I agree in most cases these situation doesn't happen, but can be)

Then, we get `array_flip(): Can only flip STRING and INTEGER values!` error at [here](https://github.com/symfony/symfony/blob/6babdb3296a5fbbd9c69d1e3410adbdf749959ef/src/Symfony/Component/Form/Extension/Core/Type/ChoiceType.php#L114).
Even if form is not `multiple`, annoying `Array to string conversion` error occurs in [here](https://github.com/symfony/symfony/blob/6babdb3296a5fbbd9c69d1e3410adbdf749959ef/src/Symfony/Component/Form/ChoiceList/ArrayChoiceList.php#L144) (via [ChoicesToValuesTransformer](https://github.com/symfony/symfony/blob/5129c4cf7e294b1a5ea30d6fec6e89b75396dcd2/src/Symfony/Component/Form/Extension/Core/DataTransformer/ChoicesToValuesTransformer.php#L74)).
(As far as I know, non-multiple and non-expanded form has no problem, thanks to [ChoiceToValueTransformer](https://github.com/symfony/symfony/blob/6babdb3296a5fbbd9c69d1e3410adbdf749959ef/src/Symfony/Component/Form/Extension/Core/DataTransformer/ChoiceToValueTransformer.php#L43))

To resolve these problems, I just added a simple-validation listener to choice type.

Commits
-------

64d7a82d [Form] Fix ChoiceType to ensure submitted data is not nested unnecessarily