[mlir][NFC] Provide accessor for TableGen record

Constraint and Predicate classes did not expose their underlying
tablegen records. This means that for Constraints, it is not possible
to get the underlying base constraint of a variadic constraint.
For Predicate, it is not possible to get the predicates forming an
`Or` predicate for instance.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D133264