fix: error on non-exhaustive enums with zero width backing type

[WillLillis]

Closes #19856

[rohlem]

Thanks for the fix!
Small nitpick, but re-testing I noticed that _ = enum(u1) {a, b, c}; results in error: enumeration value '2' too large for type 'u1'.
However, when adding _ to the fields, this compile error is overruled by error: non-exhaustive enum specifies every value.
IMO the order should be reversed, i.e. "enumeration value too large" is more important/pressing,
because if you remove _ and then get the error that the tag type is still too small,
you might increase the tag type again, in which case you might want it to be non-exhaustive again.

Just my opinion though, probably shouldn't block this PR, and it can be a separate discussion if we don't all agree on this.

[WillLillis]

Thanks for the fix!
Small nitpick, but re-testing I noticed that _ = enum(u1) {a, b, c}; results in error: enumeration value '2' too large for type 'u1'.
However, when adding _ to the fields, this compile error is overruled by error: non-exhaustive enum specifies every value. IMO the order should be reversed, i.e. "enumeration value too large" is more important/pressing, because if you remove _ and then get the error that the tag type is still too small, you might increase the tag type again, in which case you might want it to be non-exhaustive again.

Just my opinion though, probably shouldn't block this PR, and it can be a separate discussion if we don't all agree on this.

Sounds good to me!