Add a basic C API for the MLIR PassManager as well as a basic TableGen backend for creating passes

This is exposing the basic functionalities (create, nest, addPass, run) of
the PassManager through the C API in the new header: `include/mlir-c/Pass.h`.

In order to exercise it in the unit-test, a basic TableGen backend is
also provided to generate a simple C wrapper around the pass
constructor. It is used to expose the libTransforms passes to the C API.

Reviewed By: stellaraccident, ftynse

Differential Revision: https://reviews.llvm.org/D90667