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

[Kyungwoo Lee via llvm-commits]

https://github.com/kyulee-com approved this pull request.

> xxh3_64bits is a fixed algorithm and the hash values will not change.

LGTM

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