libstdc++: [_Hashtable] Use RAII type to manage rehash functor state

     Replace usage of __try/__catch with a RAII type to restore rehash 
functor
     state when needed.

     libstdc++-v3/ChangeLog:

             * include/bits/hashtable_policy.h (_RehashStateGuard): New.
             (_Insert_base<>::_M_insert_range(_IIt, _IIt, const 
_NodeGet&, false_type)):
             Adapt.
             * include/bits/hashtable.h (__rehash_guard_t): New.
             (__rehash_state): Remove.
             (_M_rehash): Remove.
             (_M_rehash_aux): Rename into _M_rehash.
             (_M_assign_elements, _M_insert_unique_node, 
_M_insert_multi_node): Adapt.
             (rehash): Adapt.


Tested under Linux x64.

Ok to commit ?

François