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487c7a754a3b93bc0f9de14bdced48007a96ae55 by Greg Falcon <gfalcon@google.com>:

Add support for absl::Hash to hash unordered containers.  These can now be hashed directly, as well as combined in AbslHashValue implementations.

This also adds a new method, `H::combine_unordered()`, to the public AbslHashValue hash state API.  This allows users to implement hash specializations for their own unordered collection types.

A traits class, `H::is_hashable<T>`, is also added to the hash state API.  H::is_hashable<T>::value reflects whether type T is considered hashable by the AbslHashValue framework.  This allows users to properly SFINAE templated versions of AbslHashValue.  (The AbslHashValue implementation added to raw_hash_set shows an example of its use.)

PiperOrigin-RevId: 422856706
GitOrigin-RevId: 487c7a754a3b93bc0f9de14bdced48007a96ae55
Change-Id: Id31fd4ccba282f8c9ae6fcee6ae0ad0f7879f456