WIP: Fix #13363
The pattern match checker doesn't handle pattern synonyms (yet); it
just regards them as additional data constructors. But that's wrong
from the perspective of redudancy checking: A match on a pattern synonym
will always lead to an overlap warning, because the mismatch case in
ConCon will always return NotCovered, which is unsound.
The proper conservative approximation is to return Covered whenever we
mismatched against a pattern synonym.
Edited by Ben Gamari