LibDNS: Add a default entry for localhost

In the future, we may want to parse /etc/hosts (or equivalent) into the
cache; this commit only adds localhost to make the normal workflow work.

Libraries/LibDNS/Resolver.h

@@ -126,6 +126,20 @@ class Resolver {
         : m_pending_lookups(make<RedBlackTree<u16, PendingLookup>>())
         , m_create_socket(move(create_socket))
     {
+        m_cache.with_write_locked([&](auto& cache) {
+            auto add_v4v6_entry = [&cache](StringView name_string, IPv4Address v4, IPv6Address v6) {
+                auto name = Messages::DomainName::from_string(name_string);
+                auto ptr = make_ref_counted<LookupResult>(name);
+                ptr->will_add_record_of_type(Messages::ResourceType::A);
+                ptr->will_add_record_of_type(Messages::ResourceType::AAAA);
+                cache.set(name_string, ptr);
+
+                ptr->add_record({ .name = {}, .type = Messages::ResourceType::A, .class_ = Messages::Class::IN, .ttl = 0, .record = Messages::Records::A { v4 }, .raw = {} });
+                ptr->add_record({ .name = {}, .type = Messages::ResourceType::AAAA, .class_ = Messages::Class::IN, .ttl = 0, .record = Messages::Records::AAAA { v6 }, .raw = {} });
+            };
+
+            add_v4v6_entry("localhost"sv, { 127, 0, 0, 1 }, IPv6Address::loopback());
+        });
     }
 
     NonnullRefPtr<Core::Promise<Empty>> when_socket_ready()