…node in an .xlf file. (leofeyer)

This PR was merged into the 2.3 branch.

Discussion
----------

[Translation][Xliff Loader] Support omitting the <target> node in an .xlf file.

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

This is a cherry pick of #15604 on 2.3

Commits
-------

3dcda1a Support omitting the <target> node in an .xlf file.

…r (stof)

This PR was merged into the 2.3 branch.

Discussion
----------

[Translation] Fix the string casting in the XliffFileLoader

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

#15611 broke the usage of Xliff translation files (and so all Symfony projects as they are used in core) because it stores SimpleXmlElement instances in the MessageCatalogue, which breaks when loading the cache:

```
PHP Fatal error:  Call to undefined method SimpleXMLElement::__set_state() in .../app/cache/test/translations/catalogue.en.1cd7e874b24ab41081c7781e4161053bf515fc91.php on line 9
```

this is how the cache looks like (truncated a lot of course):

```php

$catalogue = new MessageCatalogue('en', array (
  'validators' =>
  array (
    'This value should be false.' =>
    SimpleXMLElement::__set_state(array(
       0 => 'This value should be false.',
    )),
  ),
));
```

This is a critical bug in the 2.3 and 2.7 branches

Commits
-------

b856f62 [Translation] Fix the string casting in the XliffFileLoader