Improve EnumSanityRule

[VincentLanglet]

Closes phpstan/phpstan#13768

[VincentLanglet]

The message changed from

Enum %s is not backed, but case %s has value %s.

to

Enum %s is not backed, but case %s has value.

I'm not sure you're okay with this or if

• it should be behind bleedingEdge
• it should be behind some extra condition (and have two differents messages ?)

[VincentLanglet]

By setting enum cases value here, instead of in the condition

if ($stmt->expr instanceof Node\Scalar\Int_ || $stmt->expr instanceof Node\Scalar\String_)

I also catch other constant values like the one set with a class Constant.

Cf test

enum Backed: int {
	case One = Foo::A;
	case Two = Foo::A;
}