better type propagation for MULTIPLE-VALUE-BIND

better type propagation for MULTIPLE-VALUE-BIND

  Previously code such as

   (multiple-value-bind (x y) (known-call ...)
      ...)

  could lose the derived type for KNOWN-CALL when it was converted to
  an inline lambda: the derived type correctly ended up associated
  with the final VALUES call in the inlined code, but
  CONVERT-MV-BIND-TO-LET lost that.

  Address this by propagating the derived type of VALUES to the
  VALUES arguments.

  Allows removing the TRULY-THE kludge from the new TRUNCATE
  transform.