LibIPC: Support sending Variants over IPC

The format is quite simply the type index followed by the type in its
own native encoding; just implementing the receive side with static
typing is a bit convoluted. The only limitation of this implementation
is that the variant type has to contain an Empty somewhere as it is not
default constructible otherwise.

Co-authored-by: Ali Mohammad Pur <mpfard@serenityos.org>

Userland/Libraries/LibIPC/Decoder.cpp

@@ -208,4 +208,10 @@ ErrorOr<void> decode(Decoder& decoder, Core::ProxyData& data)
     return {};
 }
 
+// No-op.
+ErrorOr<void> Decoder::decode(AK::Empty&)
+{
+    return {};
+}
+
 }