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

[Fangrui Song via cfe-commits]

MaskRay wrote:

> > > This looks generally good.
>
> Which tests are still failing?

OK. 12c0281f8c73bc1aa20d1517357e0e12c3f8bb4e has fixed these failures. If this PR is applied as well, applying https://github.com/MaskRay/llvm-project/tree/hashing-seed will not cause `check-clang` failures.

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