Annotate a place to add more Objective-C stuff.

I think this is the Right Place to add the Objective-C receiver to the instrumentation function's parameters.
Explicitly enumerate switch values.
Clean up a few trivial things.

For instance, unused forward declarations, whitespace issues, stylistic violence done by the author of the Objective-C support... :)
Add missing "\n" in default ignored-event handler.
More robust structure-type building.

Only build named (structure) types that don't already exist and make sure the ones we build are named what we expect! The second ought to be unnecessary in the presence of the first, but assertions are good.
Implement EventTranslator::CallUpdateState.
TranslationFn::AddInstr*(Automaton, FunctionEvent)

This method adds the instrumentation required by a FunctionEvent (currently C-only, but I think I've annotated the places where Objective-C support code needs to be inserted).

This commit splits AddInstrumentation() into high-level methods that understand the semantics of program events and a low-level method that is agnostic to them but understands the details of translation function layout.
Assertion: debug stuff is available in debug mode.
Add support for calling translation functions.
Use new instrumentation API in callee contexts.

It actually works! However, we can't drop the old API yet: other kinds of instrumenters still use it.