[llvm-branch-commits] Extract SipHash implementation into a header. (PR #134197)

[Peter Collingbourne via llvm-branch-commits]

https://github.com/pcc created https://github.com/llvm/llvm-project/pull/134197

This is so that we'll be able to use it in compiler-rt as well.
Dependencies on LLVM Support were removed from the header by restoring
code from the original SipHash implementation.