[mlir][emitc] Add verification for the emitc.get_field op (#152577)
This MR adds a `verifier` for the `emitc.get_field` op.
- The `verifier` checks that the `emitc.get_field` operation is nested
inside an `emitc.class` op.
- Additionally, appropriate tests for erroneous cases were added for
class-related operations in `invalid_ops.mlir`.