LibCpp: Add support for type aliases with typedef/using

Author: https://github.com/alimpfard
Commit: 543ccecc0b
Pull-request: https://github.com/SerenityOS/serenity/pull/19447
Reviewed-by: https://github.com/AtkinsSJ

Userland/Libraries/LibCpp/AST.h

@@ -1007,6 +1007,23 @@ public:
     }
 };
 
+class TypedefDeclaration : public Declaration {
+public:
+    virtual ~TypedefDeclaration() override = default;
+    virtual StringView class_name() const override { return "TypedefDeclaration"sv; }
+    virtual void dump(FILE* = stdout, size_t indent = 0) const override;
+
+    TypedefDeclaration(ASTNode const* parent, Optional<Position> start, Optional<Position> end, DeprecatedString const& filename)
+        : Declaration(parent, start, end, filename)
+    {
+    }
+
+    void set_alias(Type const& alias) { m_alias = alias; }
+    Type const* alias() const { return m_alias.ptr(); }
+
+private:
+    RefPtr<Type const> m_alias;
+};
 template<>
 inline bool ASTNode::fast_is<Identifier>() const { return is_identifier(); }
 template<>