stage2: detect duplicate enum values

[g-w1]

Closes https://github.com/ziglang/zig/issues/12805

[Vexu]

Could you also handle this?

const T = enum(u8) {
    a = 0x12,
    b = 0x11,
    c,
};

(just add the same fix a few lines lower)

Also don't add a new case when there is already one for stage1 that can be moved instead.

[g-w1]

Thanks @Vexu for keeping up with this!