[StableHash] Implement with xxh3_64bits (#105849)
This is a follow-up to address a suggestion from
https://github.com/llvm/llvm-project/pull/105619.
The main goal of this change is to efficiently implement stable hash
functions using the xxh3 64bits API.
`stable_hash_combine_range` and `stable_hash_combine_array` functions
are removed and consolidated into a more general `stable_hash_combine`
function that takes an `ArrayRef<stable_hash>` as input.
