[clang] Mix bits in FileID::getHashValue() (PR #223794)

[Akira Hatanaka via cfe-commits]

ahatanak wrote:

Thank you.

Compile-time impact looks neutral overall. `bin/clang-24` itself changes by only +0.01%. It looks like there's a real decrease in `clang/lib/Parse/*` and `Driver/ToolChains/*`, but it's offset by a long tail of files with small increases elsewhere.

https://github.com/llvm/llvm-project/pull/223794