[libc++] Removes a GCC bug work-around.

[Mordante]

Summary

Based on the comments in [1] this should be fixed in GCC-11.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=37804

[philnik]

Why are these declarations there at all if they don't seem to be required?

libcxx/include/__tree

36–40

This looks really weird.

[Mordante]

libcxx/include/__tree

36–40

Since they are used as friends, see my comment below.

889

They are used as friends here.

libcxx/include/__tree

 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if defined(__GNUC__) && !defined(__clang__) // gcc.gnu.org/PR37804
 template <class, class, class, class> class _LIBCPP_TEMPLATE_VIS map;
 template <class, class, class, class> class _LIBCPP_TEMPLATE_VIS multimap;
 template <class, class, class> class _LIBCPP_TEMPLATE_VIS set;
 template <class, class, class> class _LIBCPP_TEMPLATE_VIS multiset;
-#endif
 
 template <class _Tp, class _Compare, class _Allocator> class __tree;
 template <class _Tp, class _NodePtr, class _DiffType>
     class _LIBCPP_TEMPLATE_VIS __tree_iterator;
 template <class _Tp, class _ConstNodePtr, class _DiffType>
     class _LIBCPP_TEMPLATE_VIS __tree_const_iterator;
 
 template <class _Pointer> class __tree_end_node;
 template <class _VoidPtr> class __tree_node_base;
private:
     _LIBCPP_INLINE_VISIBILITY
     __node_pointer __get_np() const { return static_cast<__node_pointer>(__ptr_); }
     template <class, class, class> friend class __tree;
     template <class, class, class> friend class _LIBCPP_TEMPLATE_VIS __tree_const_iterator;
     template <class> friend class _LIBCPP_TEMPLATE_VIS __map_iterator;
     template <class, class, class, class> friend class _LIBCPP_TEMPLATE_VIS map;
     template <class, class, class, class> friend class _LIBCPP_TEMPLATE_VIS multimap;
     template <class, class, class> friend class _LIBCPP_TEMPLATE_VIS set;
     template <class, class, class> friend class _LIBCPP_TEMPLATE_VIS multiset;
 };
 
 template <class _Tp, class _NodePtr, class _DiffType>
 class _LIBCPP_TEMPLATE_VIS __tree_const_iterator
 {
     typedef __tree_node_types<_NodePtr>                     _NodeTypes;
     typedef typename _NodeTypes::__node_pointer             __node_pointer;
     typedef typename _NodeTypes::__node_base_pointer        __node_base_pointer;