Lower and analyze all the flavors of the RCU API

[PappasBrent]

The RCU API comes in a variety of "flavors", and each flavor offers its own slightly different form of some of the core RCU functions/macros, and some flavors offer completely unique functions/macros that others don't (e.g., only the bh flavor offers the local_bh_disable() and local_bh_enable() functions.

Right now we mostly lower and analyze the base flavor to MLIR, but we should lower and analyze all of them.
Since all the flavors have so much in common, we should try to find an abstraction to make it easier to implement them.
Some ideas are

• Define an abstract base class for each RCU function and define concrete inheriting classes for each flavor that implements that function.
• Define an RCU flavor class and add each RCU function it offers as a component.