Allow promoted readonly properties to be reassigned once in constructor

[nicolas-grekas]

I find it quite annoying that readonly properties play badly with CPP (constructor property promotion).

Doing simple processing of any argument before assigning it to a readonly property forces opting out of CPP.

This PR allows setting once a readonly property in the body of a constructor after the property was previously set using CPP.

Before this PR (CPP not possible):

class Point {
    public readonly float $x;
    public readonly float $y;

    public function __construct(
        float $x = 0.0,
        float $y = 0.0,
    ) {
        $this->x = abs($x);
        $this->y = abs($y);
    }
}

After this PR:

class Point {
    public function __construct(
        public readonly float $x = 0.0,
        public readonly float $y = 0.0,
    ) {
        $this->x = abs($x);
        $this->y = abs($y);
    }
}

This allows keeping properties declaration in its compact form.

I'll submit an RFC of course but would also welcome early feedback here before.

[nicolas-grekas]

Implementation is now ready.
Team work with Claude Code opus 4.5 💪
The CI is green (the failures are about CI tools that couldn't be installed, unrelated to the patch)

[nicolas-grekas]

RFC published at https://wiki.php.net/rfc/promoted_readonly_constructor_reassign