Note [considerAccessible]

Consider (T18610)

  f :: Bool -> Int
  f x = case (x, x) of
    (True,  True)  -> 1
    (False, False) -> 2
    (True,  False) -> 3 -- Warning: Redundant

The third case is detected as redundant. But it may be the intent of the
programmer to keep the dead code, in order for it not to bitrot or to support
debugging scenarios. But there is no way to communicate that to the
pattern-match checker! The only way is to deactivate pattern-match checking
whole-sale, which is quite annoying. Hence, we define in "GHC.Exts":

  considerAccessible = True

'considerAccessible' is treated specially by the pattern-match checker in that a
guard with it as the scrutinee expression will keep its parent clause alive:

  g :: Bool -> Int
  g x = case (x, x) of
    (True,  True)  -> 1
    (False, False) -> 2
    (True,  False) | GHC.Exts.considerAccessible -> 3 -- No warning

The key bits of the implementation are:

  1. Its definition is recognised as known-key (see "GHC.Builtin.Names").
  2. After "GHC.HsToCore.Pmc.Desugar", the guard will end up as a 'PmCon', where
     the match var is the known-key 'considerAccessible' and the constructor
     against which it matches is 'True'.
  3. We recognise the 'PmCon' in 'GHC.HsToCore.Check.checkGrd' and inflate the
     incoming set of values for all guards downstream to the unconstrained
     'initNablas' set, e.g. /all/ values.
     (The set of values that falls through that particular guard is empty, as
     matching 'considerAccessible' against 'True' can't fail.)

Note that 'considerAccessible' breaks the invariant that incoming sets of values
reaching syntactic children are subsets of that of the syntactic ancestor:
A whole match, like that of the third clause of the example, might have no
incoming value, but its single RHS has incoming values because of (3).

That means the 'is_covered' flag computed in 'GHC.HsToCore.Pmc.cirbsMatch'
is irrelevant and should not be used to flag all children as redundant (which is
what we used to do).

We achieve great benefits with a very simple implementation.
There are caveats, though:

  (A) Putting potentially failing guards /after/ the
      'considerAccessible' guard might lead to weird check results, e.g.,

        h :: Bool -> Int
        h x = case (x, x) of
          (True,  True)  -> 1
          (False, False) -> 2
          (True,  False) | GHC.Exts.considerAccessible, False <- x -> 3
          Warning: Not matched: (_, _)

      That *is* fixable, although we would pay with a much more complicated
      implementation.
  (B) If the programmer puts a 'considerAccessible' marker on an accessible
      clause, the checker doesn't warn about it. E.g.,

        f :: Bool -> Int
        f True | considerAccessible = 0
        f False = 1

      will not emit any warning whatsoever. We could implement code that warns
      here, but it wouldn't be as simple as it is now.