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, 3 insertions, 2 deletions

diff --git a/compiler/GHC/HsToCore/Binds.hs b/compiler/GHC/HsToCore/Binds.hs
index 86d309c73d..a80b2cc2d3 100644
--- a/compiler/GHC/HsToCore/Binds.hs
+++ b/compiler/GHC/HsToCore/Binds.hs
@@ -75,6 +75,7 @@ import UniqSet( nonDetEltsUniqSet )
 import MonadUtils
 import qualified GHC.LanguageExtensions as LangExt
 import Control.Monad
+import Data.List.NonEmpty ( nonEmpty )
 
 {-**********************************************************************
 *                                                                      *
@@ -175,8 +176,8 @@ dsHsBind dflags b@(FunBind { fun_id = L _ fun
 dsHsBind dflags (PatBind { pat_lhs = pat, pat_rhs = grhss
                          , pat_ext = NPatBindTc _ ty
                          , pat_ticks = (rhs_tick, var_ticks) })
-  = do  { body_expr <- dsGuarded grhss ty
-        ; checkGuardMatches PatBindGuards grhss
+  = do  { rhss_deltas <- checkGuardMatches PatBindGuards grhss
+        ; body_expr <- dsGuarded grhss ty (nonEmpty rhss_deltas)
         ; let body' = mkOptTickBox rhs_tick body_expr
               pat'  = decideBangHood dflags pat
         ; (force_var,sel_binds) <- mkSelectorBinds var_ticks pat body'