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/LibCrypto/Checksum/ChecksumFunction.h

@@ -35,6 +35,9 @@ class ChecksumFunction {
 public:
     virtual void update(ReadonlyBytes data) = 0;
     virtual ChecksumType digest() = 0;
+
+protected:
+    virtual ~ChecksumFunction() = default;
 };
 
 }

Userland/Libraries/LibCrypto/Cipher/Cipher.h

@@ -92,6 +92,9 @@ public:
         ptr[index] = (u8)value;
     }
 
+protected:
+    virtual ~CipherBlock() = default;
+
 private:
     virtual Bytes bytes() = 0;
     PaddingMode m_padding_mode;
@@ -132,6 +135,9 @@ public:
 
     virtual String class_name() const = 0;
 
+protected:
+    virtual ~Cipher() = default;
+
 private:
     PaddingMode m_padding_mode;
 };

Userland/Libraries/LibCrypto/Hash/HashFunction.h

@@ -57,6 +57,9 @@ public:
     virtual void reset() = 0;
 
     virtual String class_name() const = 0;
+
+protected:
+    virtual ~HashFunction() = default;
 };
 }
 }

Userland/Libraries/LibCrypto/PK/Code/Code.h

@@ -48,6 +48,8 @@ public:
     HashFunction& hasher() { return m_hasher; }
 
 protected:
+    virtual ~Code() = default;
+
     HashFunction m_hasher;
 };
 

Userland/Libraries/LibCrypto/PK/PK.h

@@ -60,6 +60,8 @@ public:
     virtual size_t output_size() const = 0;
 
 protected:
+    virtual ~PKSystem() = default;
+
     PublicKeyType m_public_key;
     PrivateKeyType m_private_key;
 };