libstdc++: [_Hashtable] Add before begin bucket index cache on range insert

Add a policy to maintain a cache of the before begin bucket index in the 
context
of range insertion.

libstdc++-v3/ChangeLog:

     * include/bits/hashtable_policy.h (_CacheBbeginPolicy): New, maintain
     a cache.
     (_NoCacheBbeginPolicy): New, no cache.
     (_ReuseOrAllocNode<>): Inherit _CacheBBeginPolicy.
     (_AllocNode<>): Add cache policy template parameter.
     (_Map_base<>::operator[]): Adapt, use _NoCacheBBeginPolicy.
     (_Insert_base<>__node_gen_type): Replace by...
     (_Insert_base<>::__alloc_node_gen_t<>): ...this. Use cache policy as a
     template parameter.
     (_Insert_base<>::insert): Adapt.
     (_Insert_base<>::try_emplace): Adapt.
     (_Insert<>::__node_gen_type): Replace by...
     (_Insert<>::__alloc_node_gen_t): ...this, use _NoCacheBBeginPolicy.
     (_Insert<>::insert): Adapt.
     * include/bits/hashtable.h
     (_Hashtable<>::__alloc_node_gen_t): Remove.
     (_Hashtable<>::__alloc_node_gen_cache_bbegin_t): New.
     (_Hashtable<>::__no_cache_bbegin_policy_t): New.
     (_Hashtable<>::__cache_bbegin_policy_t): New.
     (_Hashtable<>::_CacheBBeginPolicy): Add friend declaration.
     (_Hashtable<>::_NoCacheBBeginPolicy): Add friend declaration.
     (_Hashtable<>::_M_insert_bucket_begin): Add BBegin policy.
     (_Hashtable<>::_M_insert_unique_node): Likewise.
     (_Hashtable<>::_M_insert_multi_node): Likewise.

Tested under Linux x64.

François