`Hashing.h` is non-deterministic between runs. Update the index hashing
to use BLAKE3 for the record hash. xxhash is faster in benchmarks that
I've found, but there's no easy `HashBuilder` option for it today.
This makes the tests more focused on what they are interested in testing
and allows flexibility on changing dependency printing on the clang side
without breaking the swift tests.
This converts the instances of the pattern for which we have a proper
substitution in lit. This will make it easier to replace it
appropriately with Windows equivalents.
