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.

1 files changed, 3 insertions, 0 deletions

diff --git a/Userland/Libraries/LibX86/Interpreter.h b/Userland/Libraries/LibX86/Interpreter.h
index efbf4164d8..49f71b50ce 100644
--- a/Userland/Libraries/LibX86/Interpreter.h
+++ b/Userland/Libraries/LibX86/Interpreter.h
@@ -622,6 +622,9 @@ public:
     virtual void wrap_0xD2(const Instruction&) = 0;
     virtual void wrap_0xD3_16(const Instruction&) = 0;
     virtual void wrap_0xD3_32(const Instruction&) = 0;
+
+protected:
+    virtual ~Interpreter() = default;
 };
 
 typedef void (Interpreter::*InstructionHandler)(const Instruction&);