[llvm] [Support] Add SipHash-based 64-bit stable hash function (PR #160945)

[Oliver Hunt via llvm-commits]

ojhunt wrote:

Have you verified arm64e tests are unaffected? In principle codegen should be fine due to truncation, but there's a builtin (`__builtin_ptrauth_string_discriminator`) which may fail

https://github.com/llvm/llvm-project/pull/160945