Pmc: Implement `considerAccessible` (#18610)

Consider (`T18610`):
```hs
  f :: Bool -> Int
  f x = case (x, x) of
    (True,  True)  -> 1
    (False, False) -> 2
    (True,  False) -> 3 -- Warning: Redundant
```
The third clause will be flagged as redundant. Nevertheless, the
programmer might intend to keep the clause in order to avoid bitrot.

After this patch, the programmer can write
```hs
  g :: Bool -> Int
  g x = case (x, x) of
    (True,  True)  -> 1
    (False, False) -> 2
    (True,  False) | GHC.Exts.considerAccessible -> 3 -- No warning
```
And won't be bothered any longer. See also `Note [considerAccessible]`
and the updated entries in the user's guide.

Fixes #18610 and #19228.