* don't store `has_well_defined_layout` in memory.
 * remove struct `hasWellDefinedLayout` logic. it's just
   `layout != .Auto`. This means we only need one implementation, in
   Type.
 * fix some of the cases being wrong in `hasWellDefinedLayout`, such as
   optional pointers.
 * move `tag_ty_inferred` field into a position that makes it more
   obvious how the struct layout will be done. Also we don't have a
   compiler that intelligently moves fields around so this layout is
   better.
 * Sema: don't `resolveTypeLayout` in `zirCoerceResultPtr` unless
   necessary.
 * Rename `ComptimePtrLoadKit` `target` field to `pointee` to avoid
   confusion with `target`.