[mlir] Expose Value hierarchy to C API

The Value hierarchy consists of BlockArgument and OpResult, both of which
derive Value. Introduce IsA functions and functions specific to each class,
similarly to other class hierarchies. Also, introduce functions for
pointer-comparison of Block and Operation that are necessary for testing and
are generally useful.

Reviewed By: stellaraccident, mehdi_amini

Differential Revision: https://reviews.llvm.org/D89714