[libc++] Remove outdated synopsis parts in experimental/functional

[jloser]

Summary

Remove mentions of experimental::function, its operators, etc. They are no
longer in experimental/functional.

libcxx/include/experimental/functional

 /*
    experimental/functional synopsis
 
 #include <algorithm>
 
 namespace std {
 namespace experimental {
 inline namespace fundamentals_v1 {
-
-    // See C++14 20.9.9, Function object binders
-    template <class T> constexpr bool is_bind_expression_v
-      = is_bind_expression<T>::value;
-    template <class T> constexpr int is_placeholder_v
-      = is_placeholder<T>::value;
-
-    // 4.2, Class template function
-    template<class> class function; // undefined
-    template<class R, class... ArgTypes> class function<R(ArgTypes...)>;
-
-    template<class R, class... ArgTypes>
-    void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&);
-
-    template<class R, class... ArgTypes>
-    bool operator==(const function<R(ArgTypes...)>&, nullptr_t) noexcept;
-    template<class R, class... ArgTypes>
-    bool operator==(nullptr_t, const function<R(ArgTypes...)>&) noexcept;
-    template<class R, class... ArgTypes>
-    bool operator!=(const function<R(ArgTypes...)>&, nullptr_t) noexcept;
-    template<class R, class... ArgTypes>
-    bool operator!=(nullptr_t, const function<R(ArgTypes...)>&) noexcept;
-
     // 4.3, Searchers
     template<class ForwardIterator, class BinaryPredicate = equal_to<>>
       class default_searcher;
 
     template<class RandomAccessIterator,
              class Hash = hash<typename iterator_traits<RandomAccessIterator>::value_type>,
              class BinaryPredicate = equal_to<>>
       class boyer_moore_searcher;
inline namespace fundamentals_v1 {
     boyer_moore_horspool_searcher<RandomAccessIterator, Hash, BinaryPredicate>
     make_boyer_moore_horspool_searcher(
         RandomAccessIterator pat_first, RandomAccessIterator pat_last,
         Hash hf = Hash(), BinaryPredicate pred = BinaryPredicate());
 
   } // namespace fundamentals_v1
   } // namespace experimental
 
-  template<class R, class... ArgTypes, class Alloc>
-  struct uses_allocator<experimental::function<R(ArgTypes...)>, Alloc>;
-
 } // namespace std
 
 */
 
 #include <__debug>
 #include <__memory/uses_allocator.h>
 #include <algorithm>
 #include <array>