PmCheck: Implement Long-distance information with Covered sets

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

1 files changed, 4 insertions, 4 deletions

diff --git a/compiler/typecheck/TcRnTypes.hs b/compiler/typecheck/TcRnTypes.hs
index e9da98379c..856dfa7f14 100644
--- a/compiler/typecheck/TcRnTypes.hs
+++ b/compiler/typecheck/TcRnTypes.hs
@@ -99,7 +99,7 @@ import TcOrigin
 import Annotations
 import InstEnv
 import FamInstEnv
-import {-# SOURCE #-} GHC.HsToCore.PmCheck.Types (Delta)
+import {-# SOURCE #-} GHC.HsToCore.PmCheck.Types (Deltas)
 import IOEnv
 import RdrName
 import Name
@@ -317,9 +317,9 @@ data DsLclEnv = DsLclEnv {
         dsl_loc     :: RealSrcSpan,      -- To put in pattern-matching error msgs
 
         -- See Note [Note [Type and Term Equality Propagation] in Check.hs
-        -- The oracle state Delta is augmented as we walk inwards,
-        -- through each pattern match in turn
-        dsl_delta   :: Delta
+        -- The set of reaching values Deltas is augmented as we walk inwards,
+        -- refined through each pattern match in turn
+        dsl_deltas  :: Deltas
      }
 
 -- Inside [| |] brackets, the desugarer looks