* include/bits/alloc_traits.h (__alloctr_rebind): Remove.
	(__allocator_traits_base): New base class.
	(__alloc_rebind): Reimplement in terms of detection idiom.
	(allocator_traits): Derive from __allocator_traits_base. Reimplement
	nested types in terms of detection idiom. Simplify SFINAE constraints
	on overloaded static member functions.
	* include/bits/hashtable.h (_Hashtable): Use __alloc_rebind instead
	of __alloctr_rebind.
	* testsuite/20_util/scoped_allocator/propagation.cc: Define rebind.
	* testsuite/23_containers/unordered_set/instantiation_neg.cc: Adjust
	dg-error line number.

From-SVN: r225244