Check EmptyCase by simply adding a non-void constraint
Commit message:
We can express non-void constraints since !1733 (closed), so we can now express
the strictness of -XEmptyCase just by adding a non-void constraint
to the initial Uncovered set.
For case x of {} we thus check that the Uncovered set { x | x /~ ⊥ }
is non-empty. This is conceptually simpler than the plan outlined in
#17376 (closed), because it talks to the oracle directly.
In order for this patch to pass the testsuite, I had to fix handling of newtypes in the pattern-match checker (#17248 (closed)).
Since we use a different code path (well, the main code path) for
-XEmptyCase now, we apparently also handle #13717 (closed) correctly.
There's also some dead code that we can get rid off now.
provideEvidence has been updated to provide output more in line with
the old logic, which used inhabitationCandidates under the hood.
A consequence of the shift away from the UncoveredPatterns type is
that we don't report reduced type families for empty case matches,
because the pretty printer is pure and only knows the match variable's
type.