[clang] [llvm] [Serialization] Use stable hash functions (PR #96136)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> This looks generally good.
> 
> > Some check-clang-modules tests still fail (@ChuanqiXu9)
> 
> What does this mean? Do you mean there are some failures after this patch? What are they? Or are these tests failing before this patch?

When https://github.com/MaskRay/llvm-project/tree/hashing-seed is applied, some module tests will fail.
This PR fixes most of them but some are not and I am having a hard time figuring out them...

Applying this PR alone does not cause failures.

https://github.com/llvm/llvm-project/pull/96136