swift234

Implement a Caller Analysis.

The analysis can tell all the callsites which calls a function in the module.

The analysis is computed and kept up-to-date lazily.

At the core of it, it keeps a list of functions that need to be recomputed for
the Caller/Callee relation to be precise and on every query, the analysis makes
sure to recompute them and clear the list before any query.

This is NFC right now. I am going to wire it up to function signature analysis
eventually.