re PR libstdc++/57263 (std::set with user-defined allocator - compile error)

re PR libstdc++/57263 (std::set with user-defined allocator - compile error)

	PR libstdc++/57263
	* include/bits/forward_list.h (_Fwd_list_base): Convert to/from
	allocator's pointer type.
	* include/bits/hashtable.h (_Hashtable): Likewise.
	* testsuite/util/testsuite_allocator.h (CustomPointerAlloc): Add.
	* testsuite/23_containers/forward_list/allocator/ext_ptr.cc: New.
	* testsuite/23_containers/unordered_set/allocator/ext_ptr.cc: New.
	* testsuite/23_containers/vector/allocator/ext_ptr.cc: New.

From-SVN: r200136