disallow left over bits in packed unions

[Rexicon226]

closes #19754

• Makes packed unions require a backing int type.
• Requires all fields in the packed union to have the same bit size.

The error message gathers the incorrect fields and prints them at the same time:

empty.zig:1:26: error: all fields must have a bit size of 6
const Bar = packed union(u6) {
                         ^~
empty.zig:7:8: note: field with bit size 9 here
    x: u9,
       ^~
empty.zig:8:8: note: field with bit size 64 here
    y: u64,
       ^~~

For backends, we can access the backing integer type through tagTypeUnordered, although maybe we should rename tag type to something else now that it has multiple meanings.

[Rexicon226]

I think I approached this a bit wrong, I'm gonna re-open it later when I have more time.