Adjust tr1::_Hashtable to work with std::allocator in C++20

Adjust tr1::_Hashtable to work with std::allocator in C++20

In C++20 std::allocator will no longer define construct and destroy
member functions, so using them needs to go via allocator_traits.

	* include/tr1/hashtable.h (tr1::_Hashtable::_M_allocate_node): Use
	__gnu_cxx::__alloc_traits for allocator construct function.
	(tr1::_Hashtable::_M_deallocate_node): Likewise for destroy function.

From-SVN: r276575