Description:
------------
When testing enums in 8.1 I tried to assign a constant value to a backed enum. I got a message saying that the value should be constant. That's very misleading. The RFC states:

"Equivalent values must be literals or literal expressions. Constants and constant expressions are not supported. That is, 1+1 is allowed, but 1 + SOME_CONST is not. This is primarily due to implementation complexity. (See Future Scope below.)"

I think the message should say "Enum case value must be a literal or a literal expression" or constants should be allowed just like they are in normal classes. 



Test script:
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<?php

const A = 'foo';

class Suit {
    const Heart = A;
}

enum Suit: string {
    case Heart = A;
}

Expected result:
----------------
No fatal errors or warnings

Actual result:
--------------
Fatal error: Enum case value must be constant in /in/sj7b4 on line 10