libstdc++: Unordered containers merge re-use hash code.     When merging between 2 unordered containers with same hasher we can re-use     the cached hash code if any.     Use previous insert iterator as a hint for the next insert in case of multi container.             * include/bits/hashtable_policy.h (_Hash_code_base<>::_ReuseOrComputeHash<>): New. (_Hash_code_base<>::_M_hash_code<_H2>(const _H2&, const _Hash_node_value<>&)): New.             * include/bits/hashtable.h (_Hashtable<>::_M_merge_unique): Use latter.             (_Hashtable<>::_M_merge_multi): Likewise.             * testsuite/23_containers/unordered_multiset/modifiers/merge.cc (test05): New test.             * testsuite/23_containers/unordered_set/modifiers/merge.cc (test04): New test. Tested under Linux x86_64. Ok to commit ? François