Everything: Add -Wnon-virtual-dtor flag

This flag warns on classes which have `virtual` functions but do not
have a `virtual` destructor.

This patch adds both the flag and missing destructors. The access level
of the destructors was determined by a two rules of thumb:
1. A destructor should have a similar or lower access level to that of a
   constructor.
2. Having a `private` destructor implicitly deletes the default
   constructor, which is probably undesirable for "interface" types
   (classes with only virtual functions and no data).

In short, most of the added destructors are `protected`, unless the
compiler complained about access.

Userland/Libraries/LibJS/Console.h

@@ -93,6 +93,8 @@ public:
     virtual Value count_reset() = 0;
 
 protected:
+    virtual ~ConsoleClient() = default;
+
     VM& vm();
 
     GlobalObject& global_object() { return m_console.global_object(); }

Userland/Libraries/LibJS/Runtime/Cell.h

@@ -58,6 +58,7 @@ public:
 
     protected:
         virtual void visit_impl(Cell*) = 0;
+        virtual ~Visitor() = default;
     };
 
     virtual void visit_edges(Visitor&) { }