fix the move semantic of Constrained_triangulation_plus_2

The comparison functor passed to the constraints hierarchy has a
pointer to the `Constrained_triangulation_plus_2` that contains it.
That means the defaulted move constructor and move assignment operator of `Polyline_constraint_hierarchy_2` cannot be used to
implement the move constructor and move assignment operator of
`Constrained_triangulation_plus_2`.

Add a special constructor for `Polyline_constraint_hierarchy_2`, that is
usee to move the hierarchy and reassign the comparison operator.

Triangulation_2/include/CGAL/Constrained_triangulation_plus_2.h

@@ -206,7 +206,13 @@ public:
     : Constrained_triangulation_plus_2(ctp.geom_traits())
   { copy_triangulation(ctp);}
 
-  Constrained_triangulation_plus_2(Constrained_triangulation_plus_2&&) = default;
+  Constrained_triangulation_plus_2(Constrained_triangulation_plus_2&& other_ctp) noexcept
+    : Triangulation(std::move(other_ctp))
+    , Constraint_hierarchy(std::move(other_ctp.hierarchy_ref()), Vertex_handle_compare(this))
+  {
+    // The hierarchy is moved, so the vertex handles are still valid.
+    // The triangulation is moved, so the vertex handles are still valid.
+  }
 
   ~Constrained_triangulation_plus_2() override {}
 
@@ -216,7 +222,14 @@ public:
     return *this;
   }
 
-  Constrained_triangulation_plus_2& operator=(Constrained_triangulation_plus_2&&) = default;
+  Constrained_triangulation_plus_2& operator=(Constrained_triangulation_plus_2&& other_ctp) noexcept {
+    if (this != &other_ctp) {
+      static_cast<Triangulation&>(*this) = std::move(other_ctp);
+      static_cast<Constraint_hierarchy&>(*this) =
+            Constraint_hierarchy(std::move(other_ctp.hierarchy_ref()), Vertex_handle_compare(this));
+    }
+    return *this;
+  }
 
   template<class InputIterator>
   Constrained_triangulation_plus_2(InputIterator first,

Triangulation_2/include/CGAL/Triangulation_2/internal/Polyline_constraint_hierarchy_2.h

@@ -524,13 +524,29 @@ private:
   struct Priv { // encapsulate the private members in a struct, to detect direct access to them
     Priv(Compare comp)
     : comp(comp)
-    #if CGAL_USE_BARE_STD_MAP
+#if CGAL_USE_BARE_STD_MAP
     , sc_to_c_map(Pair_compare(comp))
 #else
     , sc_to_c_map()
 #endif
     {}
 
+    Priv(const Priv&) = default;
+    Priv(Priv&&) = default;
+    Priv& operator=(const Priv&) = default;
+    Priv& operator=(Priv&&) = default;
+
+    Priv(Priv&& other, const Compare& comp) noexcept
+    : comp(comp)
+    , sc_to_c_map(std::move(other.sc_to_c_map))
+    , free_ids(std::move(other.free_ids))
+    , constraints_set(std::move(other.constraints_set))
+    {
+#if CGAL_USE_BARE_STD_MAP
+      CGAL_error_msg("This constructor cannot be used with CGAL_USE_BARE_STD_MAP");
+#endif
+    }
+
     Compare comp;
     Sc_to_c_map sc_to_c_map;
     std::stack<size_type, std::vector<size_type>> free_ids;
@@ -538,10 +554,17 @@ private:
   } priv;
 public:
   Polyline_constraint_hierarchy_2(const Compare& comp) : priv(comp) {}
+
   Polyline_constraint_hierarchy_2(const Polyline_constraint_hierarchy_2& ch) : priv(ch.priv.comp) { copy(ch); }
+
   Polyline_constraint_hierarchy_2(Polyline_constraint_hierarchy_2&&) = default;
 
+  Polyline_constraint_hierarchy_2(Polyline_constraint_hierarchy_2&& other, const Compare& comp) noexcept
+    : priv(std::move(other.priv), comp)
+  {}
+
   ~Polyline_constraint_hierarchy_2() { clear(); }
+
   void clear();
 
   Polyline_constraint_hierarchy_2& operator=(const Polyline_constraint_hierarchy_2& ch) { return copy(ch); }