libstdc++: [_Hashtable] Avoid redundant usage of rehash policy

Bypass usage of __detail::__distance_fwd and check for need to rehash 
when assigning an initializer_list to
an unordered_multimap or unordered_multiset.

libstdc++-v3/ChangeLog:

     * include/bits/hashtable_policy.h
     (_Insert_base<>::_M_insert_range(_InputIte, _InputIte, _NodeGen&)): 
New.
     (_Insert_base<>::_M_insert_range(_InputIte, _InputIte, true_type)): 
Use latter.
     (_Insert_base<>::_M_insert_range(_InputIte, _InputIte, 
false_type)): Likewise.
     * include/bits/hashtable.h
(_Hashtable<>::operator=(initializer_list<value_type>)): Likewise.
     (_Hashtable<>::_Hashtable(_InputIte, _InputIte, size_type, const 
_Hash&, const _Equal&,
     const allocator_type&, false_type)): Likewise.

Ok to commit ?

François