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

[Nikita Popov via llvm-commits]

nikic wrote:

Reverted in https://github.com/llvm/llvm-project/commit/54662112676976b57fdb67a806d4d471be0d4aa9.

@kazutakahirata Can you please address review feedback *before* you layer more patches on top?

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