AK: Explictly disallow lvalue reference types within Variant

This prevents an ICE with GCC trying to declare e.g. Variant<String&>.

Using a concept is a bit overkill here, but clang otherwise trips over
the friendship declaration to other Variant types:

    template<typename... NewTs>
    friend struct Variant;

Without using a concept, clang believes this is re-declaring the Variant
type with differing requirements ("error: requires clause differs in
template redeclaration").

AK/Variant.h

@@ -215,7 +215,10 @@ namespace AK {
 struct Empty {
 };
 
-template<typename... Ts>
+template<typename T>
+concept NotLvalueReference = !IsLvalueReference<T>;
+
+template<NotLvalueReference... Ts>
 struct Variant
     : public Detail::MergeAndDeduplicatePacks<Detail::VariantConstructors<Ts, Variant<Ts...>>...> {
 private:
@@ -244,7 +247,7 @@ public:
     {
     }
 
-    template<typename... NewTs>
+    template<NotLvalueReference... NewTs>
     friend struct Variant;
 
     Variant() requires(!can_contain<Empty>()) = delete;