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/LibX86/Instruction.h

@@ -41,6 +41,9 @@ typedef void (Interpreter::*InstructionHandler)(const Instruction&);
 class SymbolProvider {
 public:
     virtual String symbolicate(FlatPtr, u32* offset = nullptr) const = 0;
+
+protected:
+    virtual ~SymbolProvider() = default;
 };
 
 template<typename T>
@@ -316,6 +319,9 @@ public:
     virtual u16 read16() = 0;
     virtual u32 read32() = 0;
     virtual u64 read64() = 0;
+
+protected:
+    virtual ~InstructionStream() = default;
 };
 
 class SimpleInstructionStream final : public InstructionStream {