The control-flow graph and all its hierarchy components
have been moved from `model` to `efa`. The CFG is now
serialized onto the LLVM IR module as a metadata.
1. Hoist the logic to retrieve a model CallEdge from an llvm::CallInst
out of `getCallSitePrototype()`, in a separate `getCallEdge()`
function.
2. Modify `getCallSitePrototype()` so that the parent function's type is
optional.
Antonio Frighetto