genmc/schedule: Handle new GenMC scheduling API

When GenMC says that an execution should not be explored further,
Miri needs to know why (e.g., so that it returns an appropriate
exit value). Until now, some (erroneous) heuristics were used
for that.

This commit changes the Miri/GenMC API to check the scheduling
result of GenMC (that now reflects the "stop type").

src/tools/miri/genmc-sys/cpp/src/MiriInterface/Exploration.cpp

@@ -22,13 +22,23 @@ auto MiriGenmcShim::schedule_next(
     // a scheduling decision.
     threads_action_[curr_thread_id].kind = curr_thread_next_instr_kind;
 
-    if (const auto result = GenMCDriver::scheduleNext(threads_action_))
-        return SchedulingResult { ExecutionState::Ok, static_cast<int32_t>(result.value()) };
-    if (getExec().getGraph().isBlocked())
-        return SchedulingResult { ExecutionState::Blocked, 0 };
-    if (getResult().status.has_value()) // the "value" here is a `VerificationError`
-        return SchedulingResult { ExecutionState::Error, 0 };
-    return SchedulingResult { ExecutionState::Finished, 0 };
+    auto result = GenMCDriver::scheduleNext(threads_action_);
+    return std::visit(
+        [](auto&& arg) {
+            using T = std::decay_t<decltype(arg)>;
+            if constexpr (std::is_same_v<T, int>)
+                return SchedulingResult { ExecutionState::Ok, static_cast<int32_t>(arg) };
+            else if constexpr (std::is_same_v<T, Blocked>)
+                return SchedulingResult { ExecutionState::Blocked, 0 };
+            else if constexpr (std::is_same_v<T, Error>)
+                return SchedulingResult { ExecutionState::Error, 0 };
+            else if constexpr (std::is_same_v<T, Finished>)
+                return SchedulingResult { ExecutionState::Finished, 0 };
+            else
+                static_assert(false, "non-exhaustive visitor!");
+        },
+        result
+    );
 }
 
 /**** Execution start/end handling ****/