c5a27f11

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Auto-kill mutations that produce non-exhaustive patterns

Re-enable Opt_WarnIncompletePatterns (and friends) on the validation
HscEnv so deSugarExpr's coverage checker runs.  A RemoveCase that drops
the only alternative covering a constructor — e.g. the `_` wildcard in
`case xs of [] -> ...; [_] -> ...; _ -> ...` — now produces a
DsNonExhaustivePatterns message, fails the silent-desugar check in
validateAlt, and is dropped before reaching the manifest.

The compiler's incomplete-patterns warning would catch these mutations
in any normal build, so emitting them is pure noise.