[static analyzer] Factor out ArgEffect and RetEffect into public header file.

This is a WIP change to allow other clients to query the retain count
heuristics of the static analyzer.
[static analyzer] add a simple "CallEffects" API to query the retain count semantics of a method.

This is intended to be a simplified API, whose internals are
deliberately less efficient for the purpose of a simplified interface,
for use with clients that want to query the analyzer's heuristics for
determining retain count semantics.

There are no immediate clients, but it is intended to be used
by the ObjC modernizer.