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/Utilities/test-js.cpp

@@ -99,6 +99,8 @@ public:
         s_the = this;
     }
 
+    virtual ~TestRunner() = default;
+
     void run();
 
     const Test::Counts& counts() const { return m_counts; }