Also display suggestions for the commands using the eval cache

Make `nix build .#nix-armv8l-linux` work for example

src/libutil/suggestions.hh

@@ -40,4 +40,62 @@ public:
     Suggestions& operator+=(const Suggestions & other);
 };
 
+// Either a value of type `T`, or some suggestions
+template<typename T>
+class OrSuggestions {
+public:
+    using Raw = std::variant<T, Suggestions>;
+
+    Raw raw;
+
+    T* operator ->()
+    {
+        return &**this;
+    }
+
+    T& operator *()
+    {
+        if (auto elt = std::get_if<T>(&raw))
+            return *elt;
+        throw Error("Invalid access to a failed value");
+    }
+
+    operator bool() const noexcept
+    {
+        return std::holds_alternative<T>(raw);
+    }
+
+    OrSuggestions(T t)
+        : raw(t)
+    {
+    }
+
+    OrSuggestions()
+        : raw(Suggestions{})
+    {
+    }
+
+    static OrSuggestions<T> failed(const Suggestions & s)
+    {
+        auto res = OrSuggestions<T>();
+        res.raw = s;
+        return res;
+    }
+
+    static OrSuggestions<T> failed()
+    {
+        return OrSuggestions<T>::failed(Suggestions{});
+    }
+
+    const Suggestions & get_suggestions()
+    {
+        static Suggestions noSuggestions;
+        if (const auto & suggestions = std::get_if<Suggestions>(&raw))
+            return *suggestions;
+        else
+            return noSuggestions;
+    }
+
+};
+
 }