libstdc++: [_Hashtable] Fix some implementation inconsistencies

     Get rid of the different usages of the mutable keyword except in
     _Prime_rehash_policy where it is preserved for abi compatibility 
reason.

     Fix comment to explain that we need the computation of bucket index 
noexcept
     to be able to rehash the container when needed.

     For Standard instantiations through std::unordered_xxx containers 
we already
     force caching of hash code when hash functor is not noexcep so it 
is guarantied.

     The static_assert purpose in _Hashtable on _M_bucket_index is thus 
limited
     to usages of _Hashtable with exotic _Hashtable_traits.

     libstdc++-v3/ChangeLog:

             * include/bits/hashtable_policy.h 
(_NodeBuilder<>::_S_build): Remove
             const qualification on _NodeGenerator instance.
(_ReuseOrAllocNode<>::operator()(_Args&&...)): Remove const qualification.
             (_ReuseOrAllocNode<>::_M_nodes): Remove mutable.
             (_Insert_base<>::_M_insert_range): Remove _NodeGetter const 
qualification.
             (_Hash_code_base<>::_M_bucket_index(const 
_Hash_node_value<>&, size_t)):
             Simplify noexcept declaration, we already static_assert 
that _RangeHash functor
             is noexcept.
             * include/bits/hashtable.h: Rework comments. Remove const 
qualifier on
             _NodeGenerator& arguments.

Tested under Linux x64, ok to commit ?

François