[Validator] Fixed group handling in composite constraints
feature #11505 [Validator] Fixed group handling in composite constraints (webmozart)

This PR was merged into the 2.6-dev branch.

Discussion
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[Validator] Fixed group handling in composite constraints

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #4453
| License       | MIT
| Doc PR        | -

With this PR, it finally becomes possible to set the groups of nested constraints in All and Collection:

```php
/**
 * @Assert\Collection({
 *     "foo" = @Assert\NotNull(groups = "Strict"),
 *     "bar" = @Assert\NotBlank(groups = "Regular"),
 * })
 */
private $attr;
```

The group logic is implemented as described in [this comment](https://github.com/symfony/symfony/pull/4453#issuecomment-16513727).

This PR supports the implementation of #9888 which should now become fairly simple.

Commits
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f6c070d [Validator] Fixed group handling in composite constraints