[llvm] [StructuralHash] Refactor (PR #112621)

[Kyungwoo Lee via llvm-commits]

kyulee-com wrote:

> > > Why the use of stable_hash? This hash is not expected to be stable, e.g. because we may start hashing additional things into it.
> > 
> > 
> > As posted in https://discourse.llvm.org/t/rfc-global-function-merging/82608, I plan to make this structural hash stable (in the following PR). If it is preferable to maintain the existing structural hash implementation as is, I am open to creating a separate data structure or extending it through a type hierarchy.
> 
> Just to check, how "stable" is stable here? Is this just stable for a given LLVM build, or will it have to be stable across versions?

Here "stable" refers to a specific LLVM version. We previously discussed this in https://github.com/llvm/llvm-project/pull/105849#pullrequestreview-2257780906. Consequently, I've updated the comment in the header of `StableHashing.h` -- https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/ADT/StableHashing.h#L10C1-L14C9

https://github.com/llvm/llvm-project/pull/112621