[llvm] [ADT] Use a C++17 fold expression in hash_combine (NFC) (PR #159901)

[Nikita Popov via llvm-commits]

nikic wrote:

This appears to have a large negative effect when building with clang: https://llvm-compile-time-tracker.com/compare.php?from=507f394d03d8cb24c90ae4b2d28e8afae3b4088d&to=b3c7d25422355949227a12ded7515f5114db5bda&stat=instructions%3Au (Maybe related to making `buffer_ptr` a reference?)

https://github.com/llvm/llvm-project/pull/159901