minor #34990 Use ::class constants instead of __NAMESPACE__ when possible (fre5h)

This PR was merged into the 3.4 branch.

Discussion
----------

Use `::class` constants instead of `__NAMESPACE__` when possible

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | Related to #34987
| License       | MIT
| Doc PR        | no

Form component has a lot of built-in form types. Some of them were implemented from the very beginning. In most of them there is a such method

```php
    /**
     * {@inheritdoc}
     */
    public function getParent()
    {
        return __NAMESPACE__.'\TextType';
    }
```

This `getParent()` method was refactored in Symfony 2.8. The upgrade instructions are given here https://github.com/symfony/symfony/blob/2.8/UPGRADE-2.8.md#form

I think the `__NAMESPACE__.'\TextType';` expression was used because Symfony 2.8 was using `"php": ">=5.3.9"`, and the constant `::class` was added only in PHP 5.5

Now this line can be refactored into
```php
    /**
     * {@inheritdoc}
     */
    public function getParent()
    {
        return TextType::class;
    }
```

For example new form types, that were added later, already using the `::class` constant.
https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Form/Extension/Core/Type/ColorType.php#L23
https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Form/Extension/Core/Type/TelType.php#L23

So, in this pull request I propose to refactor all old form types to use `::class` constant. It will give a benefit during the future refactoring, because IDE or static analysers will find all usages of parent class. Unlike the `__NAMESPACE__.'\TextType';` line, which doesn't show the real link to the class for IDE or static analysers, and it could complicate finding all usages of parent class.

Commits
-------

32bf50abca Use `::class` constants instead of `__NAMESPACE__` when possible

Author: nicolas-grekas

Constraints/AbstractComparisonValidator.php

@@ -40,7 +40,7 @@ public function __construct(PropertyAccessorInterface $propertyAccessor = null)
     public function validate($value, Constraint $constraint)
     {
         if (!$constraint instanceof AbstractComparison) {
-            throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\AbstractComparison');
+            throw new UnexpectedTypeException($constraint, AbstractComparison::class);
         }
 
         if (null === $value) {

Constraints/AllValidator.php

@@ -26,7 +26,7 @@ class AllValidator extends ConstraintValidator
     public function validate($value, Constraint $constraint)
     {
         if (!$constraint instanceof All) {
-            throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\All');
+            throw new UnexpectedTypeException($constraint, All::class);
         }
 
         if (null === $value) {

Constraints/BicValidator.php

@@ -28,7 +28,7 @@ class BicValidator extends ConstraintValidator
     public function validate($value, Constraint $constraint)
     {
         if (!$constraint instanceof Bic) {
-            throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\Bic');
+            throw new UnexpectedTypeException($constraint, Bic::class);
         }
 
         if (null === $value || '' === $value) {

Constraints/BlankValidator.php

@@ -26,7 +26,7 @@ class BlankValidator extends ConstraintValidator
     public function validate($value, Constraint $constraint)
     {
         if (!$constraint instanceof Blank) {
-            throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\Blank');
+            throw new UnexpectedTypeException($constraint, Blank::class);
         }
 
         if ('' !== $value && null !== $value) {

Constraints/CallbackValidator.php

@@ -29,7 +29,7 @@ class CallbackValidator extends ConstraintValidator
     public function validate($object, Constraint $constraint)
     {
         if (!$constraint instanceof Callback) {
-            throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\Callback');
+            throw new UnexpectedTypeException($constraint, Callback::class);
         }
 
         $method = $constraint->callback;

Constraints/CardSchemeValidator.php

@@ -91,7 +91,7 @@ class CardSchemeValidator extends ConstraintValidator
     public function validate($value, Constraint $constraint)
     {
         if (!$constraint instanceof CardScheme) {
-            throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\CardScheme');
+            throw new UnexpectedTypeException($constraint, CardScheme::class);
         }
 
         if (null === $value || '' === $value) {

Constraints/ChoiceValidator.php

@@ -31,7 +31,7 @@ class ChoiceValidator extends ConstraintValidator
     public function validate($value, Constraint $constraint)
     {
         if (!$constraint instanceof Choice) {
-            throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\Choice');
+            throw new UnexpectedTypeException($constraint, Choice::class);
         }
 
         if (!\is_array($constraint->choices) && !$constraint->callback) {

Constraints/CollectionValidator.php

@@ -26,7 +26,7 @@ class CollectionValidator extends ConstraintValidator
     public function validate($value, Constraint $constraint)
     {
         if (!$constraint instanceof Collection) {
-            throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\Collection');
+            throw new UnexpectedTypeException($constraint, Collection::class);
         }
 
         if (null === $value) {

Constraints/CountValidator.php

@@ -26,7 +26,7 @@ class CountValidator extends ConstraintValidator
     public function validate($value, Constraint $constraint)
     {
         if (!$constraint instanceof Count) {
-            throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\Count');
+            throw new UnexpectedTypeException($constraint, Count::class);
         }
 
         if (null === $value) {

Constraints/CountryValidator.php

@@ -29,7 +29,7 @@ class CountryValidator extends ConstraintValidator
     public function validate($value, Constraint $constraint)
     {
         if (!$constraint instanceof Country) {
-            throw new UnexpectedTypeException($constraint, __NAMESPACE__.'\Country');
+            throw new UnexpectedTypeException($constraint, Country::class);
         }
 
         if (null === $value || '' === $value) {