1.0.12.3: less weakening of type-checks

1.0.12.3: less weakening of type-checks

* WEAKEN-TYPE used to return T for any union-type.  Instead, handle
  union-types implemented by backend properly.

* Also, if no supertype is found, don't replaces with T, as
  eg. oddball union types weakened to T can easily lead to heap
  corruption if the unchecked object ends up being trusted by the
  compiler. (See: WEAKEN-UNION-2 in type.impure.lisp.)