[llvm] [StableHash][NFC] Implement with xxh3_64bits (PR #105849)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

Thank you for doing this!

"stable" could mean the hash algorithm is deterministic for a specific compiler version, across different runs even on different architectures.
"stable" could also mean that the hash won't change across compiler versions (e.g. xxh3_64bits).

It might be worth clarifying it in the file-level comment. I believe we only intend this to be stable for a specific version of the compiler.

(I'm starting a 3-week vacation this Friday and will have limited availability. Happy for other reviewers to approve any follow-ups)

https://github.com/llvm/llvm-project/pull/105849