Bad diagnostic for an enum member in an enum that use another enum as base type

[dlangBugzillaToGithub]

basile-z reported this on 2024-07-25T18:00:53Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=24684

Description

For

```
enum A1 : int
{
    a,
    b,
}

enum A2 : A1
{
    c, // does not require +1
    d, // requires + 1
}  
```

The definition of A2.d causes the error

> /tmp/temp_7F4942E9C090.d(10,5): Error: comparison between different enumeration types `A2` and `A1`; If this behavior is intended consider using `std.conv.asOriginalType`

which makes no sense.

In fact the real problem is that `A2.d` violates the specification 18.1.5 (https://dlang.org/spec/enum.html#named_enums), specifically _"it is given the value of the previous EnumMember + 1_".

In consequence a better message would be that `d` value cannot be automatically defined because the base type does not support increment.