Add the ability to have "Ops" defined as small C++ classes, with some nice

properties:
 - They allow type checked dynamic casting from their base Operation.
 - They allow nice accessors for C++ clients, e.g. a "getIndex()" method on
   'dim' that returns an unsigned.
 - They work with both OperationInst/OperationStmt (once OperationStmt is
   implemented).
 - They get custom printing logic.  They will eventually get custom parsing,
   verifier, and builder logic as well.
 - Out of tree clients can register their own operation set without having to
   change MLIR core, e.g. for TensorFlow or custom target instructions.

This registers addf and dim as examples.

PiperOrigin-RevId: 203382993