Build a `SpecificConstant` (if needed) and `NameRef` instruction when
referencing the thunk target from a thunk. The former is necessary if
the impl is generic in order to call the right version of the thunk
target. This previously caused a crash in lowering.
Also add some more check testing for the interaction of thunks and
generics. This testing uncovered an unrelated bug with thunks for
generic interface functions for which I've added a TODO.
