BranchWatchTableModel: Assume Unreachable Code Truly Is

Source/Core/Common/Unreachable.h

@@ -0,0 +1,21 @@
+// Copyright 2024 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include "Common/CommonFuncs.h"
+
+namespace Common
+{
+// TODO C++23: Replace with std::unreachable.
+[[noreturn]] inline void Unreachable()
+{
+#ifdef _DEBUG
+  Crash();
+#elif defined(_MSC_VER) && !defined(__clang__)
+  __assume(false);
+#else
+  __builtin_unreachable();
+#endif
+}
+}  // namespace Common