* In semaStructFields and semaUnionFields we return error.GenericPoison
   if one of the field types ends up being generic poison.
   - This requires handling function calls and function types taking
     this into account when calling `typeRequiresComptime` on the return
     type.
 * Unrelated: I noticed using Valgrind that struct reification did not
   populate the `known_opv` field. After fixing it, the behavior tests
   run Valgrind-clean.
 * ZIR: use `@ptrCast` to cast between slices instead of exploiting
   the fact that stage1 incorrectly allows `@bitCast` between slices.
   - A future enhancement will make Zig support `@ptrCast` to directly
     cast between slices.