[Validator] Fixed LegacyValidator when only a constraint is validated
[Validator] Added ConstraintValidator::buildViolation() helper for BC with 2.4 API
bug #12031 [Validator] Fixed LegacyValidator when only a constraint is validated (webmozart)

This PR was merged into the 2.5 branch.

Discussion
----------

[Validator] Fixed LegacyValidator when only a constraint is validated

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

The 2.5-BC API is currently broken for the following use case:

```php
$validator->validate($value, $constraint);
```

This is fixed now.

Commits
-------

1d48206 [Validator] Fixed LegacyValidator when only a constraint is validated
bug #12016 [Validator] Added ConstraintValidator::buildViolation() helper for BC with the 2.4 API (webmozart)

This PR was merged into the 2.5 branch.

Discussion
----------

[Validator] Added ConstraintValidator::buildViolation() helper for BC with the 2.4 API

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

This PR adds a `buildViolation()` helper method to the base `ConstraintValidator` to remove the API checks (2.4 vs. 2.5) from the constraint validator implementations. Once the 2.4 API is removed, this helper method will be removed as well.

**Todos**

- [x] Backport changes from #12021

Commits
-------

6b0c24a [Validator] Added ConstraintValidator::buildViolation() helper for BC with 2.4 API