[llvm] [ADT,CodeGen] Remove stable_hash_combine_string (PR #100668)

[David Blaikie via llvm-commits]

dwblaikie wrote:

Seems unfortunate to have direct calls to specific hash implementations littered around the codebase next time we want to replace them with something better... 

I guess `stable` is intended to be stable across LLVM versions, etc? So it can't ever be changed? Could we add some sort of generation parameter to it and use that to pick the implementation, so that some clients can pick a newer generation?
Or add an API that matches the needs of these updated callers? What sort of determinism-but-not-stability do they need? Just something that's stable witihn a version, rather than an eternal stability that `stable` offers?

https://github.com/llvm/llvm-project/pull/100668