Support reference to constants as disjunction discriminators

[K-Phoen]

Given the following schema, cog should be able to correctly infer a discriminator and the associated discrimination mapping:

Layout: GridLayoutKind | RowsLayoutKind

#GridLayoutKindType: "GridLayout"

GridLayoutKind: {
    kind: #GridLayoutKindType & "GridLayout"
    // kind: #GridLayoutKindType // ← this should work too
    gridLayoutProperty: string
}

#RowsLayoutKindType: "RowsLayout"

RowsLayoutKind: {
    kind: #RowsLayoutKindType & "RowsLayout"
    // kind: #RowsLayoutKindType // ← this should work too
    rowsLayoutProperty: string
}

Today, the kind field in RowsLayoutKind and GridLayoutKind is not used as a discriminator because it is incorrectly parsed as a reference instead of a "constant reference".

This should be fixed and we need to make sure that once it is, we generate valid code for the resulting disjunction (especially in Go)