…n type declarations (nicolas-grekas)

This PR was merged into the 4.4 branch.

Discussion
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[ErrorHandler] make DebugClassLoader able to add return type declarations

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #33236
| License       | MIT
| Doc PR        | -

This is the current state of the patching logic I'm using in `DebugClassLoader`.
I run it after a `composer i -o` with the excluded classes emptied.
It's not perfect, it requires manual changes/review, but it did the job so far.

Patching is enabled with the `SYMFONY_PATCH_TYPE_DECLARATIONS` env var.
The value of the env var is an URL-encoded array, with the following parameters as of now:
- `force=0/1/docblock`:
  - `0` adds return types only to non-BC breaking places: internal/final/test methods;
  - `1` adds return types everywhere possible, potentially breaking BC with child classes that didn't declare the return types before;
  - `docblock` adds `@return` annotations to opt-out from deprecation notices that `DebugClassLoader` trigger otherwise - the annotation meaning: "I know a return type should be used here instead of an annotation, but I can't add it for BC reasons for now".
- `php71-compat=0/1`: to prevent using the `object` type when the code must be compatible with PHP 7.1

Commits
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72f6a97 [ErrorHandler] make DebugClassLoader able to add return type declarations

…CI (nicolas-grekas)

This PR was merged into the 4.4 branch.

Discussion
----------

Add return-types with help from DebugClassLoader in the CI

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #33228
| License       | MIT
| Doc PR        | -

I've spent a great deal of time on this PR, experimenting with adding return types to the codebase.

TL;DR: my conclusion is that we cannot make it for 5.0.

There are two reasons for this:
1. The burden this will put on the community is immense, especially when considering that third party libs must also be updated for any apps to work at all on a return-typed 5.0. Symfony must add them last, not first.
2. We need return type covariance, yet this won't be available before PHP 7.4, while 5.0 supports 7.2.

What's attached?
- ~a draft patching logic in `DebugClassLoader` to add return-type where it discovers this should be done~
- return types added automatically thanks to #33283
- ~manual fixes for situations not handled (yet, if possible at all) by that logic in `DebugClassLoader`~ #33332

What's achieved? Tests are green \o/

At this stage, I think we have to acknowledge we won't add return-types in 5.0 but prepare a serious plan to add them in 6.0.

This plan could be:
- [x] make DebugClassLoader able to automate adding return types.
- [x] in 4.4: add all possible return types that don't break BC, e.g. in `Tests` and in generated code
- [x] spot and fix places where annotations aren't accurate, add more annotations where possible.
- [x] ensure `DebugClassLoader` triggers the best possible deprecations that encourage ppl to add return-types in their libs/apps. This means we could decide to disable the current ones (see #33235) and to re-enable them in 5.1. This will also give us the time to fine-tune the tooling (item 1. on this list)

Ideally, we could reach a point where we could test branch 4.4 *with* return-types: we'd use the tooling to add them automatically in the CI job, then we'd run tests and they should be green. Let's do this?

Help Wanted, here is how:

*With PHP 7.4*, run `php .github/patch-types.php`. This will add return types everywhere possible.

Then run tests, e.g. `./phpunit src/Symfony/Component/HttpFoundation --exclude-group legacy,issue-32995`

Here are the components that fail with return types added, please help me check them all with a PR on [my fork](https://github.com/nicolas-grekas/symfony/tree/eh-return-types):
 - [x] src/Symfony/Bridge/Doctrine
 - [x] src/Symfony/Bridge/Monolog
 - [x] src/Symfony/Bridge/PhpUnit
 - [x] src/Symfony/Bridge/ProxyManager
 - [x] src/Symfony/Bridge/Twig
 - [x] src/Symfony/Bundle/DebugBundle
 - [x] src/Symfony/Bundle/FrameworkBundle
 - [x] src/Symfony/Bundle/SecurityBundle
 - [x] src/Symfony/Bundle/TwigBundle
 - [x] src/Symfony/Bundle/WebProfilerBundle
 - [x] src/Symfony/Bundle/WebServerBundle
 - [x] src/Symfony/Component/Asset
 - [x] src/Symfony/Component/BrowserKit
 - [x] src/Symfony/Component/Cache
 - [x] nicolas-grekas#28 src/Symfony/Component/Config
 - [x] src/Symfony/Component/Console
 - [x] src/Symfony/Component/CssSelector
 - [x] src/Symfony/Component/Debug
 - [x] nicolas-grekas#28 src/Symfony/Component/DependencyInjection
 - [x] src/Symfony/Component/DomCrawler
 - [x] src/Symfony/Component/Dotenv
 - [x] src/Symfony/Component/ErrorHandler
 - [x] src/Symfony/Component/ErrorRenderer
 - [x] nicolas-grekas#24 src/Symfony/Component/EventDispatcher
 - [x] src/Symfony/Component/ExpressionLanguage
 - [x] src/Symfony/Component/Filesystem
 - [x] src/Symfony/Component/Finder
 - [x] src/Symfony/Component/Form
 - [x] src/Symfony/Component/HttpClient
 - [x] src/Symfony/Component/HttpFoundation
 - [x] src/Symfony/Component/HttpKernel
 - [x] src/Symfony/Component/Inflector
 - [x] src/Symfony/Component/Intl
 - [x] src/Symfony/Component/Ldap
 - [x] src/Symfony/Component/Lock
 - [x] src/Symfony/Component/Mailer
 - [x] src/Symfony/Component/Messenger
 - [x] src/Symfony/Component/Mime
 - [x] src/Symfony/Component/OptionsResolver
 - [x] src/Symfony/Component/Process
 - [x] src/Symfony/Component/PropertyAccess
 - [x] src/Symfony/Component/PropertyInfo
 - [x] nicolas-grekas#25 src/Symfony/Component/Routing
 - [x] nicolas-grekas#26 src/Symfony/Component/Security
 - [x] src/Symfony/Component/Security/Core
 - [x] src/Symfony/Component/Security/Guard
 - [x] src/Symfony/Component/Security/Http
 - [x] nicolas-grekas#29 src/Symfony/Component/Serializer
 - [x] src/Symfony/Component/Security/Csrf
 - [x] src/Symfony/Component/Stopwatch
 - [x] src/Symfony/Component/Templating
 - [x] nicolas-grekas#27 src/Symfony/Component/Translation
 - [x] src/Symfony/Component/Validator
 - [x] src/Symfony/Component/VarDumper
 - [x] src/Symfony/Component/VarExporter
 - [x] src/Symfony/Component/WebLink
 - [x] src/Symfony/Component/Workflow
 - [x] src/Symfony/Component/Yaml
 - [x] src/Symfony/Contracts

Commits
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11149a1 Add return-types with help from DebugClassLoader in the CI