We really only need the analysis to tell whether a function has caller
inside the module or not. We do not need to know the callsites.
Remove them for now to make the analysis more memory efficient.
Add a note to indicate it can be extended.
Address the comments from 0acc0a8464
I still have not made up my mind how to handle deleted functions.
CallerAnalysis is not hooked up to anything yet.
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.
