[llvm] [Support] Add llvm::xxh3_128bits (PR #95863)

[Fangrui Song via llvm-commits]

MaskRay wrote:

LGTM, but we want an in-tree story about the planned filesystem feature.

> However, it is desirable to have a larger 128-bit hash key for use cases such as filesystem checksums where chance of collision needs to be negligible.

I've guided my simplification of xxh3 64-bit using `ld.lld --build-id=sha1`, which is sensitive to the hash function performance. Did you do similar performance tuning?

https://github.com/llvm/llvm-project/pull/95863