Sebastian Graf authored Feb 20, 2020 and Marge Bot committed Feb 27, 2020

Consider

```hs
data T = A | B | C

f :: T -> Int
f A = 1
f x = case x of
  A -> 2
  B -> 3
  C -> 4
```

Clearly, the RHS returning 2 is redundant. But we don't currently see
that, because our approximation to the covered set of the inner case
expression just picks up the positive information from surrounding
pattern matches. It lacks the context sensivity that `x` can't be `A`
anymore!

Therefore, we adopt the conceptually and practically superior approach
of reusing the covered set of a particular GRHS from an outer pattern
match. In this case, we begin checking the `case` expression with the
covered set of `f`s second clause, which encodes the information that
`x` can't be `A` anymore. After this MR, we will successfully warn about
the RHS returning 2 being redundant.

Perhaps surprisingly, this was a great simplification to the code of
both the coverage checker and the desugarer.

Found a redundant case alternative in `unix` submodule, so we have to
bump it with a fix.

Metric Decrease:
    T12227