libstdc++: Unordered containers merge re-use hash code
When merging 2 unordered containers with same hasher we can re-use the hash code from
the cache if any.
Also in the context of the merge operation on multi-container use previous insert iterator as a hint
for the next insert.
libstdc++-v3/ChangeLog:
* include/bits/hashtable_policy.h:
(_Hash_code_base<>::_M_hash_code(const _Hash&, const _Hash_node_value<_Value, true>&)): 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.