Switch from a string core to a 128-bit integral core. This should make
Fingerprints much cheaper to copy around and sets us up for a future
where we can provide alternative implementations of the ambient hashing
algorithm.
rdar://72313506
The difference with `ModuleFile` is that `ModuleFileSharedCore` provides immutable data and is independent of a particular ASTContext.
It is designed to be able to be shared across multiple `ModuleFile`s of different `ASTContext`s in a thread-safe manner.
