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/Shell/NodeVisitor.h b/Userland/Shell/NodeVisitor.h
index 8452d6d968..581443a27c 100644
--- a/Userland/Shell/NodeVisitor.h
+++ b/Userland/Shell/NodeVisitor.h
@@ -75,6 +75,9 @@ public:
     virtual void visit(const AST::VariableDeclarations*);
     virtual void visit(const AST::WriteAppendRedirection*);
     virtual void visit(const AST::WriteRedirection*);
+
+protected:
+    virtual ~NodeVisitor() = default;
 };
 
 }