* `Module.Union.getLayout`: fixes to support components of the union
   being 0 bits.
 * Implement `@typeInfo` for unions.
 * Add missing calls to `resolveTypeFields`.
 * Fix explicitly-provided union tag types passing a `Zir.Inst.Ref`
   where an `Air.Inst.Ref` was expected. We don't have any type safety
   for this; these typess are aliases.
 * Fix explicitly-provided `union(enum)` tag Values allocated to the
   wrong arena.