Ref: c205d652d1 (r934032422)
The idea is that `@psalm-pure` disallows `$this` usage in child classes,
which is not wanted, while `@psalm-mutation-free` allows it.
By using `@psalm-mutation-free`, we don't completely destroy inheritance
use-cases based on internal (immutable) state.
`DateTimeImmutable` is **almost** immutable: `DateTimeImmutable::__construct()` is in fact not a pure
method, since `new DateTimeImmutable('now')` produces a different value at each instantiation (by design).
This change makes sure that `DateTimeImmutable` loses its `@psalm-immutable` class-level marker,
preventing downstream misuse of the constructor inside otherwise referentially transparent code.
Note: only pure methods are stubbed here: all other methods declared by `DateTimeImmutable` (parent interface)
are NOT present here, and are either inferred from runtime reflection, or `CallMap*.php` definitions.
Methods are sorted in the order defined by reflection on PHP 8.1.8, at the time of writing this ( https://3v4l.org/3TGg8 ).
Following simplistic snippet was used to infer the current signature:
```php
<?php
$c = new \ReflectionClass(\DateTimeImmutable::class);
$methods = array_map(function ($m) {
return $m->getName()
. '(' . implode(',', array_map(function ($p) {
return $p->getType()
. ' $' . $p->getName()
. ($p->isOptional() ? ' = ' . var_export($p->getDefaultValue(), true) : '');
}, $m->getParameters())) . ')' . ($m->getReturnType() ? (': ' . $m->getReturnType()) : '');
}, $c->getMethods());
$properties = array_map(function ($m) {
return $m->getName();
}, $c->getProperties());
var_dump($methods, $properties);
```
