Fix move_if_noexcept usages in _Hashtable

[François Dumont]

Hi

 Â  This patch fix a minor problem with usage of std::move_if_noexcept. 
We use it to move node content if move construtor is noexcept but we 
eventually use the allocator_type::construct method which might be 
slightly different. I think it is better to check for this method 
noexcept qualification.

 Â  Moreover I have added a special overload for nodes containing a 
std::pair. It is supposed to allow move semantic in associative 
containers where Key is stored as const deleting std::pair move 
constructor. In this case we should still move the Value part.

 Â  It doesn't work for the moment because the std::pair piecewise 
constructor has no noexcept qualification. Is there any plan to add it ? 
I think adding it will force including <tuple> in stl_pair.h, is it fine ?

 Â  If this evolution is accepted I'll adapt it for _Rb_tree that has the 
same problem.

 Â  Working on this I also notice that content of initialization_list is 
not moved. Is there a plan to make initialization_list iterator type 
like move_iterator ? Should containers use 
__make_move_iterator_if_noexcept ?

 Â  Tested under Linux x86_64 normal mode.

 Â  Ok to commit this first step ?

 Â Â Â  * include/bits/stl_construct.h (__construct_object_a): New.
 Â Â Â  (struct _Construct_object_a): New, use latter.
 Â Â Â  (__move_construct_object_a): New.
 Â Â Â  (struct _Move_construct_object_a): New, use latter.
 Â Â Â  (struct _Move_construct_if_noexcept_a): New.
 Â Â Â  * include/std/tuple (__move_construct_object_a): New overload for
 Â Â Â  std::pair.
 Â Â Â  * include/ext/throw_allocator.h
 Â Â Â  (throw_allocator_base<>::construct<_Up, _Args...>): Add noexcept
 Â Â Â  qualification.
 Â Â Â  * testsuite/util/testsuite_allocator.h
 Â Â Â  (tracker_allocator<>::construct<_Up, _Args...>): Likewise.
 Â Â Â  * include/bits/hashtable_policy.h: Include <bits/stl_construct.h>
 Â Â Â  (_Hashtable_alloc<>::_M_allocate_node_f): New.
 Â Â Â  (_Hashtable_alloc<>::_M_allocate_node): Adapt, use latter.
 Â Â Â  (_AllocNode<>::operator()<_Args...>): Likewise.
 Â Â Â  (_AllocNode<_NodeAlloc, _ConstructF>): Inherit from _ConstructF.
 Â Â Â  (_AllocNode<>::_M_construct_f()): New.
 Â Â Â  (_ReuseOrAllocNode<>): Inherits from _AllocNode.
 Â Â Â  * include/bits/hashtable.h:
 Â Â Â  (_Hashtable<>::__alloc_node_gen_t): New template alias to _NodeAlloc.
 Â Â Â  (_Hashtable<>::__reuse_or_alloc_node_type): Replace with...
 Â Â Â  (_Hashtable<>::__reuse_or_alloc_node_gen_t): ...that.
 Â Â Â  (_Hashtable<>::_M_assign_elements<>): Remove _NodeGenerator parameter.
 Â Â Â  (_Hashtable<>::operator=(initializer_list<>)): Adapt.
 Â Â Â  (_Hashtable<>::operator=(const _Hashtable&)): Adapt.
 Â Â Â  (_Hashtable<>::_M_move_assign(_Hashtable&&, false_type)): Adapt.
 Â Â Â  (_Hashtable<>::_Hashtable(const _Hashtable&)): Adapt.
 Â Â Â  (_Hashtable<>::_Hashtable(const _Hashtable&, const allocator_type&)):
 Â Â Â  Adapt.
 Â Â Â  (_Hashtable<>::_Hashtable(_Hashtable&&, const allocator_type&)): Adapt.
 Â Â Â  * testsuite/23_containers/unordered_map/allocator/move_assign.cc:
 Â Â Â  (test01): Add checks.
 Â Â Â  (test02): Likewise.
 Â Â Â  * testsuite/23_containers/unordered_set/allocator/move_assign.cc:
 Â Â Â  (test04): New.

François

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