`SK_Fix` was used to indicate that solver has encountered a hole
along the current path but since there is `SK_Hole` now, increasing
`SK_Fix` no longer makes sense.
Just like in cases where both sides are dependent member types
with resolved base that can't be simplified to a concrete type
let's ignore this mismatch and mark affected type variable as a hole
because something else has to be fixed already for this to happen.
There are cases when adding missing conformance fix doesn't really
make sense like in case of `Void` and `Never` types because that
wouldn't result in a useful diagnostic.
This is a follow-up for rdar://problem/44770297
