Everywhere: Fix -Winconsistent-missing-override warnings from Clang

This option is already enabled when building Lagom, so let's enable it
for the main build too. We will no longer be surprised by Lagom Clang
CI builds failing while everything compiles locally.

Furthermore, the stronger `-Wsuggest-override` warning is enabled in
this commit, which enforces the use of the `override` keyword in all
classes, not just those which already have some methods marked as
`override`. This works with both GCC and Clang.

Userland/Shell/Shell.cpp

@@ -1671,7 +1671,7 @@ void Shell::bring_cursor_to_beginning_of_a_line() const
 bool Shell::has_history_event(StringView source)
 {
     struct : public AST::NodeVisitor {
-        virtual void visit(const AST::HistoryEvent* node)
+        virtual void visit(const AST::HistoryEvent* node) override
         {
             has_history_event = true;
             AST::NodeVisitor::visit(node);