libstdc++: [_Hashtable] Do not reuse untrusted cached hash code

On merge, reuse a merged node's possibly cached hash code only if we are on the
same type of hash and this hash is stateless.

Usage of function pointers or std::function as hash functor will prevent reusing
cached hash code.

libstdc++-v3/ChangeLog

	* include/bits/hashtable_policy.h
	(_Hash_code_base::_M_hash_code(const _Hash&, const _Hash_node_value<>&)): Remove.
	(_Hash_code_base::_M_hash_code<_H2>(const _H2&, const _Hash_node_value<>&)): Remove.
	* include/bits/hashtable.h
	(_M_src_hash_code<_H2>(const _H2&, const key_type&, const __node_value_type&)): New.
	(_M_merge_unique<>, _M_merge_multi<>): Use latter.
	* testsuite/23_containers/unordered_map/modifiers/merge.cc
	(test04, test05, test06): New test cases.