libstdc++: Fix regression in hash containers

libstdc++: Fix regression in hash containers

A recent change altered the layout of EBO-helper base classes, resulting
in an ambiguity when the hash function and equality predicate are the
same type.

This modifies the type of one of the base classes, so that we don't get
two base classes of the same type.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h (_Hash_code_base): Change
	index of _Hashtable_ebo_helper base class.
	* testsuite/23_containers/unordered_map/dup_types.cc: New test.