"Internal error looking up a definition" when assigning an enum a value from a variable

[bstrie]

This works:

fn main() {
    enum Stuff {
        Bar = 100
    }

    log(error, Bar);
}

But this doesn't:

fn main() {
    let foo = 100;

    enum Stuff {
        Bar = foo  // error: internal error looking up a definition
    }

    log(error, Bar);
}

[bstrie]

Related to #2428.

[catamorphism]

Related to #3668 too, I think (discrs are supposed to be constants, right?)

[catamorphism]

Also, reproduced as of a477c5a

[catamorphism]

This is basically the intersection of #2428 and #3668: discriminator exprs aren't getting resolved at all (#2428) and if they were, they would also have to be treated as constants correctly so that this would be a resolve error (#3668).