Fix include handling support

Can't be tested until clang 4.0.

include/cppast/cpp_entity_ref.hpp

@@ -32,13 +32,10 @@ namespace cppast
             return target_;
         }
 
-        /// \returns The [cppast::cpp_entity]() it refers to.
-        /// \requires An entity of matching kind must be registered in the [cppast::cpp_entity_index]() using the given id.
-        const T& get(const cpp_entity_index& idx) const noexcept
+        /// \returns An optional reference to the [cppast::cpp_entity]() it refers to.
+        type_safe::optional_ref<const T> get(const cpp_entity_index& idx) const noexcept
         {
             auto entity = idx.lookup(target_);
-            DEBUG_ASSERT(Predicate{}(entity.value()), detail::precondition_error_handler{},
-                         "invalid entity");
             return static_cast<const T&>(entity.value());
         }
 

include/cppast/cpp_preprocessor.hpp

@@ -74,6 +74,8 @@ namespace cppast
     class cpp_include_directive final : public cpp_entity
     {
     public:
+        static cpp_entity_kind kind() noexcept;
+
         /// \returns A newly built include directive.
         /// \notes It is not meant to be registered in the [cppast::cpp_entity_index](),
         /// as no other [cppast::cpp_entity]() can refer to it.