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

[Douglas Yung via cfe-commits]

dyung wrote:

Hi @ahatanak, this change is causing several test failures of some `clang/test/Analysis/html_diagnostics/relevant_lines` tests on the bots. Can you take a look?

- https://lab.llvm.org/buildbot/#/builders/46/builds/41475
- https://lab.llvm.org/buildbot/#/builders/174/builds/41451
- https://lab.llvm.org/buildbot/#/builders/202/builds/3093
- https://lab.llvm.org/buildbot/#/builders/190/builds/50187

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