Check EmptyCase by simply adding a non-void constraint

We can handle non-void constraints since !1733, 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, 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).

Since we use a different code path (well, the main code path) for
`-XEmptyCase` now, we apparently also handle #13717 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.

Fixes #13717, #17248, #17386